Borrowings and Financing Activities - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	May 31, 2024	Apr. 30, 2024	Jan. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Repayments in cash					$ 977,079,000	$ 0
Loss on extinguishment of debt			$ 0	$ 0	4,275,000	$ 0
Collateral	$ 6,497,180,000		6,497,180,000		6,497,180,000					$ 6,085,654,000
Residential mortgage loans held for sale | Asset Pledged as Collateral
Debt Instrument [Line Items]
Collateral	528,000,000		528,000,000		528,000,000					31,000,000
Total fixed-maturities available for sale | Fixed maturities AFS and trading securities
Debt Instrument [Line Items]
FHLB, collateral pledged	57,000,000		57,000,000		57,000,000					101,000,000
Senior Notes
Debt Instrument [Line Items]
Outstanding borrowings	1,064,718,000		1,064,718,000		$ 1,064,718,000					1,417,781,000
FHLB Advances | Minimum
Debt Instrument [Line Items]
FHLB, ratio of market value of collateral to advances					1.03
FHLB Advances | Maximum
Debt Instrument [Line Items]
FHLB, ratio of market value of collateral to advances					1.14
Mortgage Loan Financing Facilities | Flagstar Bank Master Repurchase Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity									$ 150,000,000
Mortgage Loan Financing Facilities | Goldman Sachs Master Repurchase Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity							$ 200,000,000
Mortgage Loan Financing Facilities | BMO Master Repurchase Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity								$ 400,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Fair value of amount outstanding	0		0		$ 0
Revolving Credit Facility | Unsecured Revolving Credit Facility, Expiration Date 2026
Debt Instrument [Line Items]
Current borrowing capacity	$ 275,000,000		$ 275,000,000		$ 275,000,000
Senior Notes due 2029 | Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount		$ 625,000,000
Net proceeds received		$ 617,000,000
Interest rate on senior notes	6.20%		6.20%		6.20%
Basis spread on variable rate					0.30%
Redemption price, percentage of principal amount					100.00%
Stock retention percentage required in order to make capital stock transactions	80.00%		80.00%		80.00%
Outstanding borrowings	$ 617,527,000		$ 617,527,000		$ 617,527,000					0
Senior Notes due 2025 | Senior Notes
Debt Instrument [Line Items]
Interest rate on senior notes	6.625%		6.625%		6.625%
Redemption price, percentage of principal amount		100.40%
Aggregate principal redemption amount		$ 525,000,000
Repayments in cash		527,000,000
Loss on extinguishment of debt		$ 4,000,000
Outstanding borrowings	$ 0		$ 0		$ 0					522,343,000
Senior Notes due 2024 | Senior Notes
Debt Instrument [Line Items]
Interest rate on senior notes	4.50%		4.50%		4.50%
Aggregate principal redemption amount	$ 450,000,000
Repayments in cash			$ 460,000,000
Outstanding borrowings	$ 0		$ 0		$ 0					$ 449,037,000