Investments (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) security	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) security	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,357,436,000		$ 5,357,436,000		$ 5,196,784,000
Fair Value	5,504,823,000		5,504,823,000		5,151,987,000
Other-than-temporary impairment losses			0		0
Assets held by insurance regulators	13,700,000		13,700,000		$ 13,500,000
Proceeds from sales of fixed income securities			183,600,000	$ 25,100,000
Net impairment losses recognized in earnings		$ 1,300,000	$ 100,000	1,300,000
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)			102.00%
FHLB advance collateral	$ 163,000,000.0		$ 163,000,000.0
Number of securities in unrealized loss position (in securities) | security	243		243		721
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,357,436,000		$ 5,357,436,000		$ 5,196,784,000
Gross Unrealized Gains	158,787,000		158,787,000		33,107,000
Gross Unrealized Losses	(11,400,000)		(11,400,000)		(77,904,000)
Fair Value	5,504,823,000		5,504,823,000		5,151,987,000
Gross realized gains, fixed income securities	1,200,000	100,000	2,000,000.0	200,000
Gross realized losses, fixed income securities	1,100,000	$ 600,000	2,300,000	$ 1,000,000.0
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	194,222,000		194,222,000		167,655,000
Gross Unrealized Gains	1,474,000		1,474,000		597,000
Gross Unrealized Losses	(127,000)		(127,000)		(1,076,000)
Fair Value	195,569,000		195,569,000		167,176,000
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,566,166,000		1,566,166,000		1,701,826,000
Gross Unrealized Gains	88,379,000		88,379,000		29,259,000
Gross Unrealized Losses	(648,000)		(648,000)		(10,985,000)
Fair Value	1,653,897,000		1,653,897,000		1,720,100,000
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,570,289,000		2,570,289,000		2,439,173,000
Gross Unrealized Gains	61,459,000		61,459,000		2,103,000
Gross Unrealized Losses	(3,134,000)		(3,134,000)		(40,514,000)
Fair Value	2,628,614,000		2,628,614,000		2,400,762,000
Asset backed securities (“ABS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	215,016,000		215,016,000		111,953,000
Gross Unrealized Gains	2,787,000		2,787,000		226,000
Gross Unrealized Losses	(102,000)		(102,000)		(146,000)
Fair Value	217,701,000		217,701,000		112,033,000
Residential mortgage backed securities (“RMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	213,024,000		213,024,000		189,238,000
Gross Unrealized Gains	183,000		183,000		32,000
Gross Unrealized Losses	(4,710,000)		(4,710,000)		(10,309,000)
Fair Value	208,497,000		208,497,000		178,961,000
Commercial mortgage backed securities (“CMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	268,189,000		268,189,000		276,352,000
Gross Unrealized Gains	4,450,000		4,450,000		888,000
Gross Unrealized Losses	(750,000)		(750,000)		(9,580,000)
Fair Value	271,889,000		271,889,000		267,660,000
Collateralized loan obligations (“CLO”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	330,530,000		330,530,000		310,587,000
Gross Unrealized Gains	55,000		55,000		2,000
Gross Unrealized Losses	(1,929,000)		(1,929,000)		(5,294,000)
Fair Value	$ 328,656,000		$ 328,656,000		$ 305,295,000