Investments (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,042,011,000		$ 5,042,011,000		$ 4,946,278,000
Fair Value	4,969,612,000		4,969,612,000		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			338,900,000	$ 233,200,000
Gross realized gains			400,000	900,000
Gross realized losses			3,300,000	1,100,000
Net impairment losses recognized in earnings	500,000	$ 0	$ 1,800,000	$ 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	810		810		586
Total fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,042,011,000		$ 5,042,011,000		$ 4,946,278,000
Gross Unrealized Gains	19,439,000		19,439,000		68,584,000
Gross Unrealized Losses	(91,838,000)		(91,838,000)		(31,547,000)
Fair Value	4,969,612,000		4,969,612,000		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	179,395,000		179,395,000		179,850,000
Gross Unrealized Gains	174,000		174,000		274,000
Gross Unrealized Losses	(3,353,000)		(3,353,000)		(1,278,000)
Fair Value	176,216,000		176,216,000		178,846,000
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,738,417,000		1,738,417,000		2,105,063,000
Gross Unrealized Gains	17,797,000		17,797,000		56,210,000
Gross Unrealized Losses	(21,608,000)		(21,608,000)		(8,749,000)
Fair Value	1,734,606,000		1,734,606,000		2,152,524,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,296,218,000		2,296,218,000		2,065,475,000
Gross Unrealized Gains	1,152,000		1,152,000		10,532,000
Gross Unrealized Losses	(42,305,000)		(42,305,000)		(9,169,000)
Fair Value	2,255,065,000		2,255,065,000		2,066,838,000
Asset backed securities (“ABS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	71,481,000		71,481,000		4,925,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(546,000)		(546,000)		(2,000)
Fair Value	70,935,000		70,935,000		4,923,000
Residential mortgage backed securities (“RMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	166,522,000		166,522,000		189,153,000
Gross Unrealized Gains	35,000		35,000		60,000
Gross Unrealized Losses	(11,594,000)		(11,594,000)		(7,364,000)
Fair Value	154,963,000		154,963,000		181,849,000
Commercial mortgage backed securities (“CMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	281,980,000		281,980,000		301,014,000
Gross Unrealized Gains	214,000		214,000		1,204,000
Gross Unrealized Losses	(11,974,000)		(11,974,000)		(4,906,000)
Fair Value	270,220,000		270,220,000		297,312,000
Collateralized loan obligations (“CLO”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	307,998,000		307,998,000		100,798,000
Gross Unrealized Gains	67,000		67,000		304,000
Gross Unrealized Losses	(458,000)		(458,000)		(79,000)
Fair Value	$ 307,607,000		$ 307,607,000		$ 101,023,000