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Investments - Fair Values and Gross Unrealized Gains and Losses (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 994,543 $ 924,987
Gross Unrealized Gains 24,074 1,987
Gross Unrealized Losses (1,010) (15,484)
Fair value of securities 1,017,607 911,490
U.S. Treasury securities and obligations of U.S. government agencies    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 48,193 48,171
Gross Unrealized Gains 688 35
Gross Unrealized Losses (116) (1,376)
Fair value of securities 48,765 46,830
Municipal debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 91,650 92,014
Gross Unrealized Gains 1,497 206
Gross Unrealized Losses (73) (963)
Fair value of securities 93,074 91,257
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 634,188 554,079
Gross Unrealized Gains 18,569 847
Gross Unrealized Losses (785) (11,688)
Fair value of securities 651,972 543,238
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 168,251 171,990
Gross Unrealized Gains 3,216 792
Gross Unrealized Losses (36) (1,457)
Fair value of securities 171,431 171,325
Total bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 942,282 866,254
Gross Unrealized Gains 23,970 1,880
Gross Unrealized Losses (1,010) (15,484)
Fair value of securities 965,242 852,650
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 52,261 58,733
Gross Unrealized Gains 104 107
Gross Unrealized Losses 0 0
Fair value of securities $ 52,365 $ 58,840