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Investments (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,673,367,000		$ 4,673,367,000		$ 4,724,355,000
Gross Unrealized Gains	69,116,000		69,116,000		28,628,000
Gross Unrealized Losses	(25,091,000)		(25,091,000)		(60,633,000)
Fair Value	4,717,392,000		4,717,392,000		4,692,350,000
Other than Temporary Impairment Losses, Portion in Other Comprehensive Income			$ 0		0
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)			102.00%
FHLB advance collateral	167,200,000		$ 167,200,000
Gross accumulated unrealized gain (losses) on available-for-sale securities	$ (25,100,000)		$ (25,100,000)		$ (60,600,000)
Number of securities in unrealized loss position (in securities) | security	343		343		607
Net impairment losses recognized in earnings	$ 0	$ 0	$ 0	$ 0
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,666,165,000		4,666,165,000		$ 4,717,211,000
Gross Unrealized Gains	69,066,000		69,066,000		28,620,000
Gross Unrealized Losses	(25,078,000)		(25,078,000)		(60,609,000)
Fair Value	4,710,153,000		4,710,153,000		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	104,168,000		104,168,000		73,847,000
Gross Unrealized Gains	331,000		331,000		407,000
Gross Unrealized Losses	(799,000)		(799,000)		(724,000)
Fair Value	103,700,000		103,700,000		73,530,000
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,069,592,000		2,069,592,000		2,147,458,000
Gross Unrealized Gains	50,505,000		50,505,000		20,983,000
Gross Unrealized Losses	(6,814,000)		(6,814,000)		(25,425,000)
Fair Value	2,113,283,000		2,113,283,000		2,143,016,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,865,640,000		1,865,640,000		1,756,461,000
Gross Unrealized Gains	16,151,000		16,151,000		6,059,000
Gross Unrealized Losses	(6,536,000)		(6,536,000)		(18,610,000)
Fair Value	1,875,255,000		1,875,255,000		1,743,910,000
ABS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,857,000		9,857,000		59,519,000
Gross Unrealized Gains	3,000		3,000		74,000
Gross Unrealized Losses	0		0		(28,000)
Fair Value	9,860,000		9,860,000		59,565,000
RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	198,756,000		198,756,000		231,733,000
Gross Unrealized Gains	113,000		113,000		102,000
Gross Unrealized Losses	(5,973,000)		(5,973,000)		(7,626,000)
Fair Value	192,896,000		192,896,000		224,209,000
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	308,735,000		308,735,000		327,042,000
Gross Unrealized Gains	1,517,000		1,517,000		769,000
Gross Unrealized Losses	(4,827,000)		(4,827,000)		(7,994,000)
Fair Value	305,425,000		305,425,000		319,817,000
CLOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,417,000		109,417,000		121,151,000
Gross Unrealized Gains	446,000		446,000		226,000
Gross Unrealized Losses	(129,000)		(129,000)		(202,000)
Fair Value	109,734,000		109,734,000		121,175,000
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,202,000		7,202,000		7,144,000
Gross Unrealized Gains	50,000		50,000		8,000
Gross Unrealized Losses	(13,000)		(13,000)		(24,000)
Fair Value	$ 7,239,000		$ 7,239,000		$ 7,128,000

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