Investments (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,682,148,000		$ 4,682,148,000		$ 4,724,355,000
Gross Unrealized Gains	58,055,000		58,055,000		28,628,000
Gross Unrealized Losses	(31,783,000)		(31,783,000)		(60,633,000)
Fair Value	4,708,420,000		4,708,420,000		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	165,900,000		$ 165,900,000
Gross accumulated unrealized gain (losses) on available-for-sale securities	$ (31,800,000)		$ (31,800,000)		$ (60,600,000)
Number of securities in unrealized loss position (in securities) | security	404		404		607
Net impairment losses recognized in earnings	$ 0	$ 0	$ 0	$ 0
Total debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,674,965,000		4,674,965,000		$ 4,717,211,000
Gross Unrealized Gains	58,014,000		58,014,000		28,620,000
Gross Unrealized Losses	(31,768,000)		(31,768,000)		(60,609,000)
Fair Value	4,701,211,000		4,701,211,000		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	64,043,000		64,043,000		73,847,000
Gross Unrealized Gains	364,000		364,000		407,000
Gross Unrealized Losses	(478,000)		(478,000)		(724,000)
Fair Value	63,929,000		63,929,000		73,530,000
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,131,471,000		2,131,471,000		2,147,458,000
Gross Unrealized Gains	42,983,000		42,983,000		20,983,000
Gross Unrealized Losses	(9,165,000)		(9,165,000)		(25,425,000)
Fair Value	2,165,289,000		2,165,289,000		2,143,016,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,823,823,000		1,823,823,000		1,756,461,000
Gross Unrealized Gains	12,699,000		12,699,000		6,059,000
Gross Unrealized Losses	(8,897,000)		(8,897,000)		(18,610,000)
Fair Value	1,827,625,000		1,827,625,000		1,743,910,000
ABS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,988,000		21,988,000		59,519,000
Gross Unrealized Gains	10,000		10,000		74,000
Gross Unrealized Losses	(11,000)		(11,000)		(28,000)
Fair Value	21,987,000		21,987,000		59,565,000
RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	209,874,000		209,874,000		231,733,000
Gross Unrealized Gains	78,000		78,000		102,000
Gross Unrealized Losses	(7,612,000)		(7,612,000)		(7,626,000)
Fair Value	202,340,000		202,340,000		224,209,000
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	310,997,000		310,997,000		327,042,000
Gross Unrealized Gains	1,548,000		1,548,000		769,000
Gross Unrealized Losses	(5,467,000)		(5,467,000)		(7,994,000)
Fair Value	307,078,000		307,078,000		319,817,000
CLOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	112,769,000		112,769,000		121,151,000
Gross Unrealized Gains	332,000		332,000		226,000
Gross Unrealized Losses	(138,000)		(138,000)		(202,000)
Fair Value	112,963,000		112,963,000		121,175,000
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,183,000		7,183,000		7,144,000
Gross Unrealized Gains	41,000		41,000		8,000
Gross Unrealized Losses	(15,000)		(15,000)		(24,000)
Fair Value	$ 7,209,000		$ 7,209,000		$ 7,128,000