Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	$ 1,907,865	$ 2,370,345
12 months or greater	236,802	294,916
Total investment portfolio	2,144,667	2,665,261
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(39,330)	(50,915)
12 months or greater	(9,443)	(9,718)
Total investment portfolio	(48,773)	(60,633)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	51,602	48,642
12 months or greater	0	0
Total investment portfolio	51,602	48,642
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(604)	(724)
12 months or greater	0	0
Total investment portfolio	(604)	(724)
Obligations of U.S. states and political subdivisions
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	830,978	1,136,676
12 months or greater	22,930	13,681
Total investment portfolio	853,908	1,150,357
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(18,391)	(24,918)
12 months or greater	(817)	(507)
Total investment portfolio	(19,208)	(25,425)
Corporate debt securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	802,288	915,777
12 months or greater	34,552	35,769
Total investment portfolio	836,840	951,546
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(12,157)	(16,771)
12 months or greater	(1,567)	(1,839)
Total investment portfolio	(13,724)	(18,610)
ABS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	2,998	3,366
12 months or greater	0	656
Total investment portfolio	2,998	4,022
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(19)	(28)
12 months or greater	0	0
Total investment portfolio	(19)	(28)
RMBS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	46,425	46,493
12 months or greater	162,729	171,326
Total investment portfolio	209,154	217,819
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(985)	(857)
12 months or greater	(6,938)	(6,769)
Total investment portfolio	(7,923)	(7,626)
CMBS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	165,771	205,545
12 months or greater	16,453	38,587
Total investment portfolio	182,224	244,132
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(6,994)	(7,529)
12 months or greater	(112)	(465)
Total investment portfolio	(7,106)	(7,994)
CLOs
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	7,276	13,278
12 months or greater	0	34,760
Total investment portfolio	7,276	48,038
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(168)	(73)
12 months or greater	0	(129)
Total investment portfolio	(168)	(202)
Equity securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	527	568
12 months or greater	138	137
Total investment portfolio	665	705
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(12)	(15)
12 months or greater	(9)	(9)
Total investment portfolio	$ (21)	$ (24)