Investments - Additional Information (Detail)		3 Months Ended			9 Months Ended
		Sep. 30, 2026 USD ($)	Mar. 31, 2026 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2026 USD ($)	Sep. 30, 2025 USD ($)	Jun. 30, 2026 USD ($)	Dec. 31, 2025 USD ($)	Jul. 01, 2025 USD ($)	Jun. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Schedule of Investments
Other Long-term Investments		$ 1,664,100,000			$ 1,664,100,000			$ 1,670,400,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans		16,400,000			16,400,000			15,900,000
Accrued Investment Income		751,300,000			751,300,000			596,000,000.0
Financing Receivable, Modified, Weighted-Average Term Increase from Modification						24 months
Repossessed Assets		0			0			0
Commitments to Fund Commercial Mortgage Loans		11,100,000			11,100,000
Real Estate Investment Property, Net		60,800,000			60,800,000			41,700,000
Real Estate Held-for-sale		$ 5,000,000.0			$ 5,000,000.0			41,900,000
Repurchase agreements - Typical Days outstanding					30
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral		102.00%			102.00%
Gross Securities Loaned		$ 99,200,000			$ 99,200,000			106,100,000
Securities Received as Collateral		40,300,000			40,300,000			34,100,000
Cash Collateral for Borrowed Securities		62,900,000			62,900,000			76,100,000
Outstanding Repurchase Agreements		0			$ 0			0
Financing Receivable, Modified in Period, to Total Financing Receivables, Percentage					0.70%
Federal Home Loan Bank Common Stock		50,400,000			$ 50,400,000			40,700,000
Gain (Loss) on Investments		(853,000,000.0)		$ 128,000,000.0	(863,200,000)	$ (96,500,000)
Impairment Loss		760,900,000		0	772,000,000.0	160,900,000
Operating Segments
Schedule of Investments
Gain (Loss) on Investments		(853,000,000.0)		128,000,000.0	(863,200,000)	(96,500,000)
2026 Fortitude Re Reinsurance Transaction | Operating Segments
Schedule of Investments
Gain (Loss) on Investments		(833,500,000)			(833,500,000)
Impairment Loss		754,400,000			754,400,000
2025 Fortitude Re Reinsurance Transaction
Schedule of Investments
Accrued Investment Income									$ 47,100,000
2025 Fortitude Re Reinsurance Transaction | Operating Segments
Schedule of Investments
Gain (Loss) on Investments			$ (46,800,000)	137,600,000		(46,800,000)
Subsidiary Dividend Funding
Schedule of Investments
Gain (Loss) on Investments						19,100,000
February 2026 FABL
Schedule of Investments
FABL maximum borrowing capacity		500,000,000.0			500,000,000.0
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	[1]	2,069,600,000			2,069,600,000			2,125,400,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	16,400,000		15,600,000	16,400,000	15,600,000	$ 16,000,000.0	15,900,000		$ 15,600,000	$ 16,100,000
Accrued Investment Income		6,700,000			6,700,000			6,700,000
Financing Receivable, Modified in Period, Amount					0	14,200,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing		0		1,000,000	0	1,000,000		0
Write-offs	[1]	0		$ (800,000)	0	$ (800,000)
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments								0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,448,100,000			1,448,100,000			$ 1,456,300,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		$ 182,600,000			$ 182,600,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		999			999
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		702			702
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		87			87
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		19			19

[1] Loan-to-Value Ratio utilizes the most recent internal valuation of the property