Investments (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) security	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,229,535,000		$ 5,196,784,000
Fair Value	5,287,360,000		5,151,987,000
Other-than-temporary impairment losses	0		0
Assets held by insurance regulators	13,600,000		$ 13,500,000
Proceeds from sales of fixed income securities	106,000,000	$ 10,800,000
Gross realized gains, fixed income securities	700,000	100,000
Net impairment losses recognized in earnings	$ 100,000	0
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)	102.00%
FHLB advance collateral	$ 171,600,000
Number of securities in unrealized loss position (in securities) | security	420		721
Total fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 5,229,535,000		$ 5,196,784,000
Gross Unrealized Gains	85,641,000		33,107,000
Gross Unrealized Losses	(27,816,000)		(77,904,000)
Fair Value	5,287,360,000		5,151,987,000
Gross realized gain, fixed income securities	1,300,000	$ 300,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	167,821,000		167,655,000
Gross Unrealized Gains	910,000		597,000
Gross Unrealized Losses	(589,000)		(1,076,000)
Fair Value	168,142,000		167,176,000
Obligations of U.S. states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,611,092,000		1,701,826,000
Gross Unrealized Gains	58,372,000		29,259,000
Gross Unrealized Losses	(2,226,000)		(10,985,000)
Fair Value	1,667,238,000		1,720,100,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,442,354,000		2,439,173,000
Gross Unrealized Gains	23,363,000		2,103,000
Gross Unrealized Losses	(9,648,000)		(40,514,000)
Fair Value	2,456,069,000		2,400,762,000
Asset backed securities (“ABS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	217,469,000		111,953,000
Gross Unrealized Gains	1,045,000		226,000
Gross Unrealized Losses	(33,000)		(146,000)
Fair Value	218,481,000		112,033,000
Residential mortgage backed securities (“RMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	188,201,000		189,238,000
Gross Unrealized Gains	143,000		32,000
Gross Unrealized Losses	(7,859,000)		(10,309,000)
Fair Value	180,485,000		178,961,000
Commercial mortgage backed securities (“CMBS”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	272,074,000		276,352,000
Gross Unrealized Gains	1,808,000		888,000
Gross Unrealized Losses	(4,599,000)		(9,580,000)
Fair Value	269,283,000		267,660,000
Collateralized loan obligations (“CLO”)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	330,524,000		310,587,000
Gross Unrealized Gains	0		2,000
Gross Unrealized Losses	(2,862,000)		(5,294,000)
Fair Value	$ 327,662,000		$ 305,295,000