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Investments - Fair Values and Gross Unrealized Gains and Losses on Investments (Details) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 316,639 $ 337,718
Gross Unrealized Gains 867 1,139
Gross Unrealized (Losses) (3,051) (2,356)
Fair Value 314,455 336,501
U.S. Treasury securities and obligations of U.S. government agencies    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 68,243 68,911
Gross Unrealized Gains 86 7
Gross Unrealized (Losses) (106) (573)
Fair Value 68,223 68,345
Municipal bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 11,001 12,009
Gross Unrealized Gains 7 27
Gross Unrealized (Losses) (24) (73)
Fair Value 10,984 11,963
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 179,650 200,358
Gross Unrealized Gains 486 883
Gross Unrealized (Losses) (2,793) (1,456)
Fair Value 177,343 199,785
Asset-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 57,745 56,440
Gross Unrealized Gains 288 222
Gross Unrealized (Losses) (128) (254)
Fair Value $ 57,905 $ 56,408