Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	$ 1,094,307	$ 2,370,345
12 months or greater	389,823	294,916
Total investment portfolio	1,484,130	2,665,261
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(12,633)	(50,915)
12 months or greater	(12,458)	(9,718)
Total investment portfolio	(25,091)	(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	74,136	48,642
12 months or greater	22,423	0
Total investment portfolio	96,559	48,642
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(524)	(724)
12 months or greater	(275)	0
Total investment portfolio	(799)	(724)
Obligations of U.S. states and political subdivisions
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	411,460	1,136,676
12 months or greater	85,700	13,681
Total investment portfolio	497,160	1,150,357
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(4,520)	(24,918)
12 months or greater	(2,294)	(507)
Total investment portfolio	(6,814)	(25,425)
Corporate debt securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	434,056	915,777
12 months or greater	75,592	35,769
Total investment portfolio	509,648	951,546
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(4,367)	(16,771)
12 months or greater	(2,169)	(1,839)
Total investment portfolio	(6,536)	(18,610)
ABS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	2,613	3,366
12 months or greater	0	656
Total investment portfolio	2,613	4,022
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	0	(28)
12 months or greater	0	0
Total investment portfolio	0	(28)
RMBS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	37,676	46,493
12 months or greater	148,748	171,326
Total investment portfolio	186,424	217,819
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(713)	(857)
12 months or greater	(5,260)	(6,769)
Total investment portfolio	(5,973)	(7,626)
CMBS
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	126,709	205,545
12 months or greater	57,189	38,587
Total investment portfolio	183,898	244,132
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(2,375)	(7,529)
12 months or greater	(2,452)	(465)
Total investment portfolio	(4,827)	(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	(129)	(73)
12 months or greater	0	(129)
Total investment portfolio	(129)	(202)
Equity securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	381	568
12 months or greater	171	137
Total investment portfolio	552	705
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(5)	(15)
12 months or greater	(8)	(9)
Total investment portfolio	$ (13)	$ (24)