Investments (Details) $ in Thousands	3 Months Ended
Mar. 31, 2022 USD ($) security	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,088,948		$ 6,397,658
Fair Value	5,935,916		6,587,581
Assets held by insurance regulatory requirements	12,800		13,400
Assets held in trust for the benefit of contractual counterparties	$ 183,400		189,800
FHLB advance, collateral pledged			$ 167,200
Number of securities in unrealized loss position (in securities) \| security	909		610
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,088,948		$ 6,397,658
Gross Unrealized Gains	53,131		218,390
Gross Unrealized Losses	(206,163)		(28,467)
Fair Value	5,935,916		6,587,581
Gains on sales	4,134	$ 3,016
Losses on sales	(4,657)	(392)
Change in credit allowance	0	18
Proceeds from sales of fixed income securities	216,824	$ 56,827
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	120,223		133,990
Gross Unrealized Gains	70		285
Gross Unrealized Losses	(4,712)		(868)
Fair Value	115,581		133,407
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,383,899		2,408,688
Gross Unrealized Gains	38,260		133,361
Gross Unrealized Losses	(90,246)		(7,396)
Fair Value	2,331,913		2,534,653
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,572,570		2,704,586
Gross Unrealized Gains	14,382		75,172
Gross Unrealized Losses	(84,660)		(13,776)
Fair Value	2,502,292		2,765,982
ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	117,517		150,888
Gross Unrealized Gains	91		830
Gross Unrealized Losses	(3,464)		(1,008)
Fair Value	114,144		150,710
RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	259,849		309,991
Gross Unrealized Gains	56		2,397
Gross Unrealized Losses	(11,594)		(3,278)
Fair Value	248,311		309,110
CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	298,490		315,330
Gross Unrealized Gains	267		5,736
Gross Unrealized Losses	(9,440)		(1,936)
Fair Value	289,317		319,130
CLOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	331,914		360,436
Gross Unrealized Gains	5		609
Gross Unrealized Losses	(1,670)		(106)
Fair Value	330,249		360,939
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,486		13,749
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(377)		(99)
Fair Value	$ 4,109		$ 13,650

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