Investments (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,974,616,000		$ 4,946,278,000
Fair Value	4,930,063,000		4,983,315,000
Other than Temporary Impairment Losses, Portion in Other Comprehensive Income	0		$ 0
Proceeds from sales of fixed income securities	10,844,000	$ 33,980,000
Gross realized gains	100,000	200,000
Gross realized losses	$ 300,000	300,000
Number of securities in unrealized loss position (in securities) | security	788		586
Net impairment losses recognized in earnings	$ 0	$ 0
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)	102.00%
FHLB advance collateral	$ 165,600,000
Total fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,974,616,000		$ 4,946,278,000
Gross Unrealized Gains	31,036,000		68,584,000
Gross Unrealized Losses	(75,589,000)		(31,547,000)
Fair Value	4,930,063,000		4,983,315,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	191,018,000		179,850,000
Gross Unrealized Gains	256,000		274,000
Gross Unrealized Losses	(2,212,000)		(1,278,000)
Fair Value	189,062,000		178,846,000
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,093,901,000		2,105,063,000
Gross Unrealized Gains	27,926,000		56,210,000
Gross Unrealized Losses	(19,130,000)		(8,749,000)
Fair Value	2,102,697,000		2,152,524,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,087,977,000		2,065,475,000
Gross Unrealized Gains	1,921,000		10,532,000
Gross Unrealized Losses	(33,819,000)		(9,169,000)
Fair Value	2,056,079,000		2,066,838,000
Asset backed securities (“ABS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,451,000		4,925,000
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(29,000)		(2,000)
Fair Value	9,422,000		4,923,000
Residential mortgage backed securities (“RMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	182,050,000		189,153,000
Gross Unrealized Gains	48,000		60,000
Gross Unrealized Losses	(10,558,000)		(7,364,000)
Fair Value	171,540,000		181,849,000
Commercial mortgage backed securities (“CMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	302,434,000		301,014,000
Gross Unrealized Gains	722,000		1,204,000
Gross Unrealized Losses	(9,800,000)		(4,906,000)
Fair Value	293,356,000		297,312,000
Collateralized loan obligations (“CLO”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	107,785,000		100,798,000
Gross Unrealized Gains	163,000		304,000
Gross Unrealized Losses	(41,000)		(79,000)
Fair Value	$ 107,907,000		$ 101,023,000