Investments (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,473,893,000		$ 5,473,893,000		$ 5,196,784,000
Fair Value	5,660,992,000		5,660,992,000		5,151,987,000
Other-than-temporary impairment losses			0		0
Assets held by insurance regulators	13,800,000		13,800,000		$ 13,500,000
Proceeds from sales of fixed income securities			201,400,000	$ 338,900,000
Net impairment losses recognized in earnings		$ 500,000	$ 100,000	1,800,000
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent)			102.00%
FHLB advance collateral	$ 165,400,000		$ 165,400,000
Number of securities in unrealized loss position (in securities) | security	183		183		721
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,473,893,000		$ 5,473,893,000		$ 5,196,784,000
Gross Unrealized Gains	195,485,000		195,485,000		33,107,000
Gross Unrealized Losses	(8,386,000)		(8,386,000)		(77,904,000)
Fair Value	5,660,992,000		5,660,992,000		5,151,987,000
Gross realized gains, fixed income securities	3,300,000	300,000	5,300,000	400,000
Gross realized losses, fixed income securities	700,000	$ 2,300,000	3,000,000.0	$ 3,300,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	197,748,000		197,748,000		167,655,000
Gross Unrealized Gains	1,456,000		1,456,000		597,000
Gross Unrealized Losses	(75,000)		(75,000)		(1,076,000)
Fair Value	199,129,000		199,129,000		167,176,000
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,552,283,000		1,552,283,000		1,701,826,000
Gross Unrealized Gains	108,763,000		108,763,000		29,259,000
Gross Unrealized Losses	(494,000)		(494,000)		(10,985,000)
Fair Value	1,660,552,000		1,660,552,000		1,720,100,000
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,653,124,000		2,653,124,000		2,439,173,000
Gross Unrealized Gains	74,630,000		74,630,000		2,103,000
Gross Unrealized Losses	(3,151,000)		(3,151,000)		(40,514,000)
Fair Value	2,724,603,000		2,724,603,000		2,400,762,000
Asset backed securities (“ABS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	212,792,000		212,792,000		111,953,000
Gross Unrealized Gains	2,819,000		2,819,000		226,000
Gross Unrealized Losses	(65,000)		(65,000)		(146,000)
Fair Value	215,546,000		215,546,000		112,033,000
Residential mortgage backed securities (“RMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	248,098,000		248,098,000		189,238,000
Gross Unrealized Gains	1,022,000		1,022,000		32,000
Gross Unrealized Losses	(2,663,000)		(2,663,000)		(10,309,000)
Fair Value	246,457,000		246,457,000		178,961,000
Commercial mortgage backed securities (“CMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	280,364,000		280,364,000		276,352,000
Gross Unrealized Gains	6,714,000		6,714,000		888,000
Gross Unrealized Losses	(266,000)		(266,000)		(9,580,000)
Fair Value	286,812,000		286,812,000		267,660,000
Collateralized loan obligations (“CLOs”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	329,484,000		329,484,000		310,587,000
Gross Unrealized Gains	81,000		81,000		2,000
Gross Unrealized Losses	(1,672,000)		(1,672,000)		(5,294,000)
Fair Value	$ 327,893,000		$ 327,893,000		$ 305,295,000