Investments (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) security	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) security	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,597,554,000		$ 5,597,554,000		$ 5,562,550,000
Fair Value	5,862,262,000		5,862,262,000		5,737,892,000
Credit allowance on available for sale fixed income securities	0		0
Other-than-temporary impairment losses recorded in other comprehensive income					0
Assets held by insurance regulators	14,200,000		$ 14,200,000		$ 13,900,000
Collateral, market value to principal balance of FHLB (as a percent)			102.00%
FHLB advance, collateral pledged	$ 165,800,000		$ 165,800,000
Number of securities in unrealized loss position (in securities) | security	156		156		217
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 5,597,554,000		$ 5,597,554,000		$ 5,562,550,000
Gross Unrealized Gains	284,035,000		284,035,000		185,244,000
Gross Unrealized Losses	(19,327,000)		(19,327,000)		(9,902,000)
Fair Value	5,862,262,000		5,862,262,000		5,737,892,000
Proceeds from sales of fixed income securities			448,500,000	$ 183,600,000
Gross realized gains, fixed income securities	7,300,000	$ 1,200,000	12,400,000	2,000,000.0
Gross realized losses, fixed income securities	3,700,000	$ 1,100,000	5,000,000.0	2,300,000
Impairment charges recognized on securities with the intent to sell	0		300,000
Net impairment losses recognized in earnings				$ 100,000
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	346,881,000		346,881,000		195,176,000
Gross Unrealized Gains	1,777,000		1,777,000		1,237,000
Gross Unrealized Losses	(41,000)		(41,000)		(210,000)
Fair Value	348,617,000		348,617,000		196,203,000
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,666,017,000		1,666,017,000		1,555,394,000
Gross Unrealized Gains	124,251,000		124,251,000		99,328,000
Gross Unrealized Losses	(1,291,000)		(1,291,000)		(857,000)
Fair Value	1,788,977,000		1,788,977,000		1,653,865,000
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,514,526,000		2,514,526,000		2,711,910,000
Gross Unrealized Gains	137,465,000		137,465,000		76,220,000
Gross Unrealized Losses	(5,160,000)		(5,160,000)		(3,008,000)
Fair Value	2,646,831,000		2,646,831,000		2,785,122,000
Asset backed securities (“ABS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	198,238,000		198,238,000		227,376,000
Gross Unrealized Gains	4,034,000		4,034,000		2,466,000
Gross Unrealized Losses	(2,716,000)		(2,716,000)		(178,000)
Fair Value	199,556,000		199,556,000		229,664,000
Residential mortgage backed securities (“RMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	293,790,000		293,790,000		271,384,000
Gross Unrealized Gains	6,055,000		6,055,000		429,000
Gross Unrealized Losses	(353,000)		(353,000)		(3,227,000)
Fair Value	299,492,000		299,492,000		268,586,000
Commercial mortgage backed securities (“CMBS”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	264,857,000		264,857,000		274,234,000
Gross Unrealized Gains	10,453,000		10,453,000		5,531,000
Gross Unrealized Losses	(1,411,000)		(1,411,000)		(779,000)
Fair Value	273,899,000		273,899,000		278,986,000
Collateralized loan obligations (“CLOs”)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	313,245,000		313,245,000		327,076,000
Gross Unrealized Gains	0		0		33,000
Gross Unrealized Losses	(8,355,000)		(8,355,000)		(1,643,000)
Fair Value	$ 304,890,000		$ 304,890,000		$ 325,466,000