Investments (Details)	1 Months Ended	12 Months Ended
	Feb. 29, 2016	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($)
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		$ 4,717,211,000	$ 4,684,148,000
Total at end of period		4,685,222,000	4,657,561,000
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair Value		7,128,000	5,645,000
Total Investment Portfolio [Abstract]
Amortized Cost		4,724,355,000	4,689,773,000
Gross Unrealized Gains		28,628,000	41,273,000
Gross Unrealized Losses		(60,633,000)	(67,840,000)
Fair Value		$ 4,692,350,000	4,663,206,000
Collateral, principal balance of FHLB (as a percent)	102.00%	102.00%
Collateral, principal balance of FHLB		$ 164,400,000
Maturities, Amortized Cost [Abstract]
Due in one year or less		433,464,000
Due after one year through five years		1,211,034,000
Due after five years through ten years		1,157,091,000
Due after ten years		1,176,177,000
Total debt securities with single maturity date		3,977,766,000
Total at end of period		4,717,211,000	4,684,148,000
Maturities, Fair Value [Abstract]
Due in one year or less		433,770,000
Due after one year through five years		1,212,650,000
Due after five years through ten years		1,139,552,000
Due after ten years		1,174,484,000
Total debt securities with single maturity date		3,960,456,000
Total at end of period		4,685,222,000	4,657,561,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		2,370,345,000	2,209,765,000
12 months or greater		294,916,000	491,195,000
Total investment portfolio		2,665,261,000	2,700,960,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(50,915,000)	(49,346,000)
12 months or greater		(9,718,000)	(18,494,000)
Total investment portfolio		$ (60,633,000)	$ (67,840,000)
Number of securities in unrealized loss position | security		607	303
For securities in an unrealized loss position, percentage of fair value to amortized cost (in hundredths)		98.00%
For securities in an unrealized loss position, percentage backed by the U.S. Government (in hundredths)		14.00%
Recognized other than temporary impairments		$ 0	$ 0	$ 144,000
Assets held by insurance regulatory requirements		13,600,000	18,900,000
Total debt securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		4,717,211,000	4,684,148,000
Gross Unrealized Gains		28,620,000	41,235,000
Gross Unrealized Losses		(60,609,000)	(67,822,000)
Total at end of period		4,685,222,000	4,657,561,000
Total Investment Portfolio [Abstract]
Fair Value		4,685,222,000	4,657,561,000
Maturities, Amortized Cost [Abstract]
Total at end of period		4,717,211,000	4,684,148,000
Maturities, Fair Value [Abstract]
Total at end of period		4,685,222,000	4,657,561,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		73,847,000	160,393,000
Gross Unrealized Gains		407,000	2,133,000
Gross Unrealized Losses		(724,000)	(1,942,000)
Total at end of period		73,530,000	160,584,000
Total Investment Portfolio [Abstract]
Fair Value		73,530,000	160,584,000
Maturities, Amortized Cost [Abstract]
Total at end of period		73,847,000	160,393,000
Maturities, Fair Value [Abstract]
Total at end of period		73,530,000	160,584,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		48,642,000	60,548,000
12 months or greater		0	1,923,000
Total investment portfolio		48,642,000	62,471,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(724,000)	(1,467,000)
12 months or greater		0	(475,000)
Total investment portfolio		(724,000)	(1,942,000)
Obligations of U.S. states and political subdivisions
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		2,147,458,000	1,766,407,000
Gross Unrealized Gains		20,983,000	33,410,000
Gross Unrealized Losses		(25,425,000)	(7,290,000)
Total at end of period		2,143,016,000	1,792,527,000
Total Investment Portfolio [Abstract]
Fair Value		2,143,016,000	1,792,527,000
Maturities, Amortized Cost [Abstract]
Total at end of period		2,147,458,000	1,766,407,000
Maturities, Fair Value [Abstract]
Total at end of period		2,143,016,000	1,792,527,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		1,136,676,000	417,615,000
12 months or greater		13,681,000	37,014,000
Total investment portfolio		1,150,357,000	454,629,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(24,918,000)	(6,404,000)
12 months or greater		(507,000)	(886,000)
Total investment portfolio		(25,425,000)	(7,290,000)
Corporate debt securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		1,756,461,000	2,046,697,000
Gross Unrealized Gains		6,059,000	2,836,000
Gross Unrealized Losses		(18,610,000)	(44,770,000)
Total at end of period		1,743,910,000	2,004,763,000
Total Investment Portfolio [Abstract]
Fair Value		1,743,910,000	2,004,763,000
Maturities, Amortized Cost [Abstract]
Total at end of period		1,756,461,000	2,046,697,000
Maturities, Fair Value [Abstract]
Total at end of period		1,743,910,000	2,004,763,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		915,777,000	1,470,628,000
12 months or greater		35,769,000	114,982,000
Total investment portfolio		951,546,000	1,585,610,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(16,771,000)	(38,519,000)
12 months or greater		(1,839,000)	(6,251,000)
Total investment portfolio		(18,610,000)	(44,770,000)
ABS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		59,519,000	116,764,000
Gross Unrealized Gains		74,000	56,000
Gross Unrealized Losses		(28,000)	(203,000)
Total at end of period		59,565,000	116,617,000
Total Investment Portfolio [Abstract]
Fair Value		59,565,000	116,617,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		59,519,000
Total at end of period		59,519,000	116,764,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		59,565,000
Total at end of period		59,565,000	116,617,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		3,366,000	86,604,000
12 months or greater		656,000	5,546,000
Total investment portfolio		4,022,000	92,150,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(28,000)	(173,000)
12 months or greater		0	(30,000)
Total investment portfolio		(28,000)	(203,000)
RMBS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		231,733,000	265,879,000
Gross Unrealized Gains		102,000	161,000
Gross Unrealized Losses		(7,626,000)	(8,392,000)
Total at end of period		224,209,000	257,648,000
Total Investment Portfolio [Abstract]
Fair Value		224,209,000	257,648,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		231,733,000
Total at end of period		231,733,000	265,879,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		224,209,000
Total at end of period		224,209,000	257,648,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		46,493,000	35,064,000
12 months or greater		171,326,000	209,882,000
Total investment portfolio		217,819,000	244,946,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(857,000)	(312,000)
12 months or greater		(6,769,000)	(8,080,000)
Total investment portfolio		(7,626,000)	(8,392,000)
CMBS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		327,042,000	237,304,000
Gross Unrealized Gains		769,000	162,000
Gross Unrealized Losses		(7,994,000)	(3,975,000)
Total at end of period		319,817,000	233,491,000
Total Investment Portfolio [Abstract]
Fair Value		319,817,000	233,491,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		327,042,000
Total at end of period		327,042,000	237,304,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		319,817,000
Total at end of period		319,817,000	233,491,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		205,545,000	134,488,000
12 months or greater		38,587,000	69,927,000
Total investment portfolio		244,132,000	204,415,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(7,529,000)	(2,361,000)
12 months or greater		(465,000)	(1,614,000)
Total investment portfolio		(7,994,000)	(3,975,000)
CLOs
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		121,151,000	61,345,000
Gross Unrealized Gains		226,000	3,000
Gross Unrealized Losses		(202,000)	(1,148,000)
Total at end of period		121,175,000	60,200,000
Total Investment Portfolio [Abstract]
Fair Value		121,175,000	60,200,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		121,151,000
Total at end of period		121,151,000	61,345,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		121,175,000
Total at end of period		121,175,000	60,200,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		13,278,000	0
12 months or greater		34,760,000	51,750,000
Total investment portfolio		48,038,000	51,750,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(73,000)	0
12 months or greater		(129,000)	(1,148,000)
Total investment portfolio		(202,000)	(1,148,000)
Debt securities issued by foreign sovereign governments
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period			29,359,000
Gross Unrealized Gains			2,474,000
Gross Unrealized Losses			(102,000)
Total at end of period			31,731,000
Total Investment Portfolio [Abstract]
Fair Value			31,731,000
Maturities, Amortized Cost [Abstract]
Total at end of period			29,359,000
Maturities, Fair Value [Abstract]
Total at end of period			31,731,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months			4,463,000
12 months or greater			0
Total investment portfolio			4,463,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months			(102,000)
12 months or greater			0
Total investment portfolio			(102,000)
Equity securities
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Amortized Cost		7,144,000	5,625,000
Gross Unrealized Gains		8,000	38,000
Gross Unrealized Losses		(24,000)	(18,000)
Fair Value		7,128,000	5,645,000
Total Investment Portfolio [Abstract]
Fair Value		7,128,000	5,645,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		568,000	355,000
12 months or greater		137,000	171,000
Total investment portfolio		705,000	526,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(15,000)	(8,000)
12 months or greater		(9,000)	(10,000)
Total investment portfolio		$ (24,000)	$ (18,000)