Investments - Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Loaned securities (Notes 5 and 6)	$ 103,996	$ 57,499
Total fixed-maturities available for sale	5,367,729	5,393,623
Allowance for Credit Losses	0	948
Total fixed-maturities available for sale	5,517,078	5,723,340
Fixed-maturities available for sale \| U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	221,407	176,033
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,719	1,677
Gross Unrealized Losses	(1,396)	(3,611)
Fair Value	221,730	174,099
Fixed-maturities available for sale \| State and municipal obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	162,964	149,258
Allowance for Credit Losses	0	0
Gross Unrealized Gains	14,694	16,113
Gross Unrealized Losses	(401)	(100)
Fair Value	177,257	165,271
Fixed-maturities available for sale \| Corporate bonds and notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,867,063	2,832,350
Allowance for Credit Losses	0	(948)	$ 0
Gross Unrealized Gains	133,665	250,771
Gross Unrealized Losses	(24,886)	(3,758)
Fair Value	2,975,842	3,078,415
Fixed-maturities available for sale \| RMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	697,581	799,814
Allowance for Credit Losses	0	0
Gross Unrealized Gains	14,313	34,439
Gross Unrealized Losses	(6,777)	(314)
Fair Value	705,117	833,939
Fixed-maturities available for sale \| CMBS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	690,827	645,071
Allowance for Credit Losses	0	0
Gross Unrealized Gains	21,444	39,495
Gross Unrealized Losses	(3,068)	(3,301)
Fair Value	709,203	681,265
Fixed-maturities available for sale \| CLO
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	529,906	569,173
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,032	2,026
Gross Unrealized Losses	(898)	(2,641)
Fair Value	530,040	568,558
Fixed-maturities available for sale \| Other ABS
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	210,657	249,988
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,142	2,901
Gross Unrealized Losses	(612)	(432)
Fair Value	211,187	252,457
Fixed-maturities available for sale \| Foreign government and agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,109	5,100
Allowance for Credit Losses	0	0
Gross Unrealized Gains	187	338
Gross Unrealized Losses	0	0
Fair Value	5,296	5,438
Fixed-maturities available for sale \| Mortgage insurance-linked notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	45,384
Allowance for Credit Losses	0
Gross Unrealized Gains	1,633
Gross Unrealized Losses	0
Fair Value	47,017
Fixed-maturities available for sale \| Total fixed-maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,430,898	5,426,787
Total fixed-maturities available for sale	5,367,729	5,393,623
Allowance for Credit Losses	0	(948)
Gross Unrealized Gains	189,829	347,760
Gross Unrealized Losses	(38,038)	(14,157)
Fair Value	5,582,689	5,759,442
Total fixed-maturities available for sale	5,517,078	5,723,340
Fixed-maturities available for sale \| Loaned securities, amortized cost \| Total fixed-maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Loaned securities (Notes 5 and 6)	63,169	33,164
Fixed-maturities available for sale \| Loaned securities
Debt Securities, Available-for-sale [Line Items]
Loaned securities (Notes 5 and 6)	65,600
Fixed-maturities available for sale \| Loaned securities \| Total fixed-maturities available for sale
Debt Securities, Available-for-sale [Line Items]
Loaned securities (Notes 5 and 6)	$ 65,611	$ 36,102

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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