Stock-based compensation expense recognized in our Consolidated Statements of Income for the years ended December 31, 2013, 2012 and 2011 is based on RSUs and options ultimately expected to vest, and has been reduced for estimated forfeitures. Estimates for forfeiture rates are based upon historical experience and are evaluated quarterly. We expect our forfeiture rate for stock option awards to be approximately 2.5% annually. We estimated a 0% forfeiture rate for our RSUs and restricted stock due to the limited number of recipients and historical experience for these awards. 4 Investments At December 31, 2013, we held the following securities and investments, recorded at either fair value or cost: At December 31, 2012, we held the following securities and investments, recorded at either fair value or cost: ADTRAN 2013 Annual Report 52 As of December 31, 2013, corporate and municipal fixed-rate bonds had the following contractual maturities: (In thousands) Corporate  bonds Municipal  fixed-rate  bonds Less than one year $55,396 $46,606 One to two years 37,838 25,284 Two to three years 70,447 55,358 Three to five years 3,178 9,054 Total $166,859 $136,302 (In thousands) Amortized Cost Gross Unrealized  Gains Gross Unrealized  Losses Fair Value/ Carrying  Value Deferred compensation plan assets $10,688 $846 $(7) $11,527 Corporate bonds 185,464 966 (18) 186,412 Municipal fixed-rate bonds 174,530 627 (73) 175,084 Municipal variable rate demand notes 34,375 — — 34,375 Fixed income bond fund 444 12 — 456 Marketable equity securities 20,966 14,630 (392) 35,204 Available-for-sale securities held at fair value $426,467 $17,081 $(490) $443,058 Restricted investment held at cost 48,250 Other investments held at cost 1,902 Total carrying value of available-for-sale investments $493,210 (In thousands) Amortized Cost Gross Unrealized  Gains Gross Unrealized  Losses Fair Value/ Carrying  Value Deferred compensation plan assets $12,300 $2,847 $(24) $15,123 Corporate bonds 166,370 534 (45) 166,859 Municipal fixed-rate bonds 135,773 583 (54) 136,302 Municipal variable rate demand notes 8,310 — — 8,310 Marketable equity securities 24,654 13,975 (177) 38,452 Available-for-sale securities held at fair value $347,407 $17,939 $(300) $365,046 Restricted investment held at cost 48,250 Other investments held at cost 1,689 Total carrying value of available-for-sale investments $414,985