Investments (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,087,857		$ 6,087,857		$ 6,397,658
Fair Value	5,712,820		5,712,820		6,587,581
Assets held by insurance regulatory requirements	12,600		12,600		13,400
Assets held in trust for the benefit of contractual counterparties	177,600		177,600		189,800
FHLB advance, collateral pledged					167,200
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,087,857		6,087,857		6,397,658
Gross Unrealized Gains	12,589		12,589		218,390
Gross Unrealized Losses	(387,626)		(387,626)		(28,467)
Fair Value	5,712,820		5,712,820		6,587,581
Gains on sales	987	$ 2,275	5,121	$ 5,291
Losses on sales	(1,093)	(353)	(5,750)	(745)
Change in credit allowance	0	31	0	49
Proceeds from sales of fixed income securities	46,730	$ 83,235	263,554	$ 140,062
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	123,595		123,595		133,990
Gross Unrealized Gains	19		19		285
Gross Unrealized Losses	(6,295)		(6,295)		(868)
Fair Value	117,319		117,319		133,407
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,455,698		2,455,698		2,408,688
Gross Unrealized Gains	11,529		11,529		133,361
Gross Unrealized Losses	(174,093)		(174,093)		(7,396)
Fair Value	2,293,134		2,293,134		2,534,653
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,532,263		2,532,263		2,704,586
Gross Unrealized Gains	945		945		75,172
Gross Unrealized Losses	(158,155)		(158,155)		(13,776)
Fair Value	2,375,053		2,375,053		2,765,982
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	108,064		108,064		150,888
Gross Unrealized Gains	9		9		830
Gross Unrealized Losses	(4,805)		(4,805)		(1,008)
Fair Value	103,268		103,268		150,710
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	240,109		240,109		309,991
Gross Unrealized Gains	31		31		2,397
Gross Unrealized Losses	(17,591)		(17,591)		(3,278)
Fair Value	222,549		222,549		309,110
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	282,090		282,090		315,330
Gross Unrealized Gains	50		50		5,736
Gross Unrealized Losses	(15,529)		(15,529)		(1,936)
Fair Value	266,611		266,611		319,130
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	337,823		337,823		360,436
Gross Unrealized Gains	6		6		609
Gross Unrealized Losses	(10,418)		(10,418)		(106)
Fair Value	327,411		327,411		360,939
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,486		4,486		13,749
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	(740)		(740)		(99)
Fair Value	3,746		3,746		$ 13,650
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,729		3,729
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Fair Value	$ 3,729		$ 3,729

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

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