Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	$ 1,552,082	$ 2,370,345
12 months or greater	208,040	294,916
Total investment portfolio	1,760,122	2,665,261
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(23,210)	(50,915)
12 months or greater	(8,573)	(9,718)
Total investment portfolio	(31,783)	(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	52,769	48,642
12 months or greater	993	0
Total investment portfolio	53,762	48,642
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(471)	(724)
12 months or greater	(7)	0
Total investment portfolio	(478)	(724)
Obligations of U.S. states and political subdivisions
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	622,767	1,136,676
12 months or greater	17,400	13,681
Total investment portfolio	640,167	1,150,357
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(8,595)	(24,918)
12 months or greater	(570)	(507)
Total investment portfolio	(9,165)	(25,425)
Corporate debt securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	649,134	915,777
12 months or greater	27,241	35,769
Total investment portfolio	676,375	951,546
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(7,664)	(16,771)
12 months or greater	(1,233)	(1,839)
Total investment portfolio	(8,897)	(18,610)
ABS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	3,362	3,366
12 months or greater	0	656
Total investment portfolio	3,362	4,022
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(11)	(28)
12 months or greater	0	0
Total investment portfolio	(11)	(28)
RMBS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	43,815	46,493
12 months or greater	155,030	171,326
Total investment portfolio	198,845	217,819
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(885)	(857)
12 months or greater	(6,727)	(6,769)
Total investment portfolio	(7,612)	(7,626)
CMBS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	172,505	205,545
12 months or greater	7,237	38,587
Total investment portfolio	179,742	244,132
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(5,439)	(7,529)
12 months or greater	(28)	(465)
Total investment portfolio	(5,467)	(7,994)
CLOs
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	7,275	13,278
12 months or greater	0	34,760
Total investment portfolio	7,275	48,038
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(138)	(73)
12 months or greater	0	(129)
Total investment portfolio	(138)	(202)
Equity securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	455	568
12 months or greater	139	137
Total investment portfolio	594	705
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(7)	(15)
12 months or greater	(8)	(9)
Total investment portfolio	$ (15)	$ (24)