Investments (Details) $ in Thousands		1 Months Ended
		Feb. 29, 2016	Mar. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			$ 4,512,387	$ 4,689,773
Gross Unrealized Gains			81,884	41,273
Gross Unrealized Losses			(30,068)	(67,840)	[1]
Fair Value			4,564,203	4,663,206
Federal Home Loan Bank advances, collateral, value of principal, percent		102.00%
FHLB advance collateral			164,600
Gross accumulated unrealized gain (losses) on available-for-sale securities			(30,100)	(67,800)
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			587,119	2,209,765
12 months or greater			735,737	491,195
Total investment portfolio			1,322,856	2,700,960
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(10,859)	(49,346)
12 months or greater			(19,209)	(18,494)
Total investment portfolio			$ (30,068)	$ (67,840)
Number of securities in unrealized loss position | security			316	303
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			$ 147,189	$ 160,393
Gross Unrealized Gains			2,474	2,133
Gross Unrealized Losses	[1]		(1,135)	(1,942)
Fair Value			148,528	160,584
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			26,586	60,548
12 months or greater			2,998	1,923
Total investment portfolio			29,584	62,471
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(1,094)	(1,467)
12 months or greater			(41)	(475)
Total investment portfolio			(1,135)	(1,942)
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			1,854,959	1,766,407
Gross Unrealized Gains			60,677	33,410
Gross Unrealized Losses	[1]		(2,506)	(7,290)
Fair Value			1,913,130	1,792,527
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			105,443	417,615
12 months or greater			60,616	37,014
Total investment portfolio			166,059	454,629
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(763)	(6,404)
12 months or greater			(1,743)	(886)
Total investment portfolio			(2,506)	(7,290)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			1,856,376	2,046,697
Gross Unrealized Gains			16,315	2,836
Gross Unrealized Losses	[1]		(18,484)	(44,770)
Fair Value			1,854,207	2,004,763
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			350,566	1,470,628
12 months or greater			348,277	114,982
Total investment portfolio			698,843	1,585,610
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(7,932)	(38,519)
12 months or greater			(10,552)	(6,251)
Total investment portfolio			(18,484)	(44,770)
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			110,241	116,764
Gross Unrealized Gains			176	56
Gross Unrealized Losses	[1]		(89)	(203)
Fair Value			110,328	116,617
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			29,107	86,604
12 months or greater			11,019	5,546
Total investment portfolio			40,126	92,150
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(42)	(173)
12 months or greater			(47)	(30)
Total investment portfolio			(89)	(203)
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			255,344	265,879
Gross Unrealized Gains			452	161
Gross Unrealized Losses	[1]		(5,155)	(8,392)
Fair Value			250,641	257,648
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			14,548	35,064
12 months or greater			204,585	209,882
Total investment portfolio			219,133	244,946
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(103)	(312)
12 months or greater			(5,052)	(8,080)
Total investment portfolio			(5,155)	(8,392)
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			220,719	237,304
Gross Unrealized Gains			1,678	162
Gross Unrealized Losses	[1]		(1,701)	(3,975)
Fair Value			220,696	233,491
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			60,778	134,488
12 months or greater			56,126	69,927
Total investment portfolio			116,904	204,415
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(925)	(2,361)
12 months or greater			(776)	(1,614)
Total investment portfolio			(1,701)	(3,975)
Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			61,350	61,345
Gross Unrealized Gains			25	3
Gross Unrealized Losses	[1]		(991)	(1,148)
Fair Value			60,384	60,200
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			0	0
12 months or greater			51,907	51,750
Total investment portfolio			51,907	51,750
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			0	0
12 months or greater			(991)	(1,148)
Total investment portfolio			(991)	(1,148)
Debt securities issued by foreign sovereign governments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost				29,359
Gross Unrealized Gains				2,474
Gross Unrealized Losses	[1]			(102)
Fair Value				31,731
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)
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			4,506,178	4,684,148
Gross Unrealized Gains			81,797	41,235
Gross Unrealized Losses			(30,061)	(67,822)	[1]
Fair Value			4,557,914	4,657,561
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			6,209	5,625
Gross Unrealized Gains			87	38
Gross Unrealized Losses	[1]		(7)	(18)
Fair Value			6,289	5,645
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			91	355
12 months or greater			209	171
Total investment portfolio			300	526
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			0	(8)
12 months or greater			(7)	(10)
Total investment portfolio			$ (7)	$ (18)

[1] At March 31, 2016 and December 31, 2015, there were no other-than-temporary impairment losses recorded in other comprehensive income.