Investments (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2017 USD ($) security	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,662,620,000		$ 4,724,355,000
Gross Unrealized Gains	35,434,000		28,628,000
Gross Unrealized Losses	(48,773,000)		(60,633,000)
Fair Value	4,649,281,000		4,692,350,000
Other-than-temporary impairments	$ 0		0
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)	102.00%
FHLB advance collateral	$ 165,000,000
Gross accumulated unrealized gain (losses) on available-for-sale securities	$ (48,800,000)		$ (60,600,000)
Number of securities in unrealized loss position (in securities) \| security	495		607
Net impairment losses recognized in earnings	$ 0	$ 0
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,655,457,000		$ 4,717,211,000
Gross Unrealized Gains	35,414,000		28,620,000
Gross Unrealized Losses	(48,752,000)		(60,609,000)
Fair Value	4,642,119,000		4,685,222,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	61,035,000		73,847,000
Gross Unrealized Gains	387,000		407,000
Gross Unrealized Losses	(604,000)		(724,000)
Fair Value	60,818,000		73,530,000
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,161,765,000		2,147,458,000
Gross Unrealized Gains	25,893,000		20,983,000
Gross Unrealized Losses	(19,208,000)		(25,425,000)
Fair Value	2,168,450,000		2,143,016,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,753,470,000		1,756,461,000
Gross Unrealized Gains	7,594,000		6,059,000
Gross Unrealized Losses	(13,724,000)		(18,610,000)
Fair Value	1,747,340,000		1,743,910,000
ABS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,107,000		59,519,000
Gross Unrealized Gains	29,000		74,000
Gross Unrealized Losses	(19,000)		(28,000)
Fair Value	37,117,000		59,565,000
RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	220,739,000		231,733,000
Gross Unrealized Gains	93,000		102,000
Gross Unrealized Losses	(7,923,000)		(7,626,000)
Fair Value	212,909,000		224,209,000
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	300,185,000		327,042,000
Gross Unrealized Gains	1,038,000		769,000
Gross Unrealized Losses	(7,106,000)		(7,994,000)
Fair Value	294,117,000		319,817,000
CLOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,156,000		121,151,000
Gross Unrealized Gains	380,000		226,000
Gross Unrealized Losses	(168,000)		(202,000)
Fair Value	121,368,000		121,175,000
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,163,000		7,144,000
Gross Unrealized Gains	20,000		8,000
Gross Unrealized Losses	(21,000)		(24,000)
Fair Value	$ 7,162,000		$ 7,128,000

Federal Home Loan Bank Advances, Collateral, Market Value Of Principal, Percent

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of the collateral pledged in support of federal home loan bank advances outstanding.

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Amount after tax of other than temporary impairment loss (OTTI) on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss and attributable to parent entity.

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