Investments - Securities In Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	$ 198,945	$ 2,209,765
12 months or greater	485,623	491,195
Total investment portfolio	684,568	2,700,960
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(4,116)	(49,346)
12 months or greater	(9,309)	(18,494)
Total investment portfolio	(13,425)	(67,840)
Debt securities issued by foreign sovereign governments
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		4,463
12 months or greater		0
Total investment portfolio		4,463
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(102)
12 months or greater		0
Total investment portfolio		(102)
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	8,996	60,548
12 months or greater	0	1,923
Total investment portfolio	8,996	62,471
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(806)	(1,467)
12 months or greater	0	(475)
Total investment portfolio	(806)	(1,942)
Obligations of U.S. states and political subdivisions
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	30,347	417,615
12 months or greater	15,842	37,014
Total investment portfolio	46,189	454,629
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(303)	(6,404)
12 months or greater	(228)	(886)
Total investment portfolio	(531)	(7,290)
Corporate debt securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	110,610	1,470,628
12 months or greater	167,209	114,982
Total investment portfolio	277,819	1,585,610
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(2,341)	(38,519)
12 months or greater	(4,381)	(6,251)
Total investment portfolio	(6,722)	(44,770)
Asset-backed securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	13,440	86,604
12 months or greater	8,047	5,546
Total investment portfolio	21,487	92,150
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(45)	(173)
12 months or greater	(5)	(30)
Total investment portfolio	(50)	(203)
Residential mortgage-backed securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	2,383	35,064
12 months or greater	203,939	209,882
Total investment portfolio	206,322	244,946
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(68)	(312)
12 months or greater	(3,505)	(8,080)
Total investment portfolio	(3,573)	(8,392)
Commercial mortgage-backed securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	33,169	134,488
12 months or greater	38,382	69,927
Total investment portfolio	71,551	204,415
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(553)	(2,361)
12 months or greater	(338)	(1,614)
Total investment portfolio	(891)	(3,975)
Collateralized loan obligations
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	0	0
12 months or greater	52,050	51,750
Total investment portfolio	52,050	51,750
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	0	0
12 months or greater	(849)	(1,148)
Total investment portfolio	(849)	(1,148)
Equity securities
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	0	355
12 months or greater	154	171
Total investment portfolio	154	526
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	0	(8)
12 months or greater	(3)	(10)
Total investment portfolio	$ (3)	$ (18)