Investments (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,983,355,000		$ 4,983,355,000		$ 4,946,278,000
Fair Value	4,926,247,000		4,926,247,000		4,983,315,000
Other than Temporary Impairment Losses, Portion in Other Comprehensive Income			0		0
Assets held by insurance regulators	13,400,000		13,400,000		$ 13,600,000
Proceeds from sales of fixed income securities			25,100,000	$ 166,600,000
Gross realized gains			200,000	800,000
Gross realized losses			$ 1,000,000	1,000,000
Net impairment losses recognized in earnings	1,300,000	$ 0		$ 0
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)			102.00%
FHLB advance collateral	$ 166,100,000		$ 166,100,000
Number of securities in unrealized loss position (in securities) | security	867		867		586
Total fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 4,983,355,000		$ 4,983,355,000		$ 4,946,278,000
Gross Unrealized Gains	27,939,000		27,939,000		68,584,000
Gross Unrealized Losses	(85,047,000)		(85,047,000)		(31,547,000)
Fair Value	4,926,247,000		4,926,247,000		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	135,248,000		135,248,000		179,850,000
Gross Unrealized Gains	210,000		210,000		274,000
Gross Unrealized Losses	(2,688,000)		(2,688,000)		(1,278,000)
Fair Value	132,770,000		132,770,000		178,846,000
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,056,724,000		2,056,724,000		2,105,063,000
Gross Unrealized Gains	26,099,000		26,099,000		56,210,000
Gross Unrealized Losses	(16,686,000)		(16,686,000)		(8,749,000)
Fair Value	2,066,137,000		2,066,137,000		2,152,524,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,137,543,000		2,137,543,000		2,065,475,000
Gross Unrealized Gains	1,174,000		1,174,000		10,532,000
Gross Unrealized Losses	(42,759,000)		(42,759,000)		(9,169,000)
Fair Value	2,095,958,000		2,095,958,000		2,066,838,000
Asset backed securities (“ABS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	71,625,000		71,625,000		4,925,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(333,000)		(333,000)		(2,000)
Fair Value	71,292,000		71,292,000		4,923,000
Residential mortgage backed securities (“RMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	174,255,000		174,255,000		189,153,000
Gross Unrealized Gains	41,000		41,000		60,000
Gross Unrealized Losses	(10,570,000)		(10,570,000)		(7,364,000)
Fair Value	163,726,000		163,726,000		181,849,000
Commercial mortgage backed securities (“CMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	294,839,000		294,839,000		301,014,000
Gross Unrealized Gains	351,000		351,000		1,204,000
Gross Unrealized Losses	(11,847,000)		(11,847,000)		(4,906,000)
Fair Value	283,343,000		283,343,000		297,312,000
Collateralized loan obligations (“CLO”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,121,000		113,121,000		100,798,000
Gross Unrealized Gains	64,000		64,000		304,000
Gross Unrealized Losses	(164,000)		(164,000)		(79,000)
Fair Value	$ 113,021,000		$ 113,021,000		$ 101,023,000