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Investments (Schedule of Available-for-sale Securities) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Investment [Line Items]    
Amortized Cost $ 1,558,457 $ 1,709,922
Gross Unrealized Gains 1,219 1,030
Gross Unrealized Losses (3,200) (3,887)
Fair Value 1,556,476 1,707,065
Agency securities [Member]    
Investment [Line Items]    
Amortized Cost 479,435 411,963
Gross Unrealized Gains 625 699
Gross Unrealized Losses (1,293) (1,169)
Fair Value 478,767 411,493
Corporate securities [Member]    
Investment [Line Items]    
Amortized Cost 805,137 842,887
Gross Unrealized Gains 509 193
Gross Unrealized Losses (1,164) (2,114)
Fair Value 804,482 840,966
Municipal securities [Member]    
Investment [Line Items]    
Amortized Cost 3,965 9,989
Gross Unrealized Gains 12 3
Gross Unrealized Losses 0 (4)
Fair Value 3,977 9,988
Government securities [Member]    
Investment [Line Items]    
Amortized Cost 269,920 445,083
Gross Unrealized Gains 73 135
Gross Unrealized Losses (743) (600)
Fair Value $ 269,250 $ 444,618