Investments - Additional Information (Detail)		6 Months Ended
		Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)	Mar. 31, 2026 USD ($)	Dec. 31, 2025 USD ($)	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Schedule of Investments
Other Long-term Investments		$ 1,628,800,000			$ 1,670,400,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans		16,000,000.0			15,900,000
Accrued Investment Income		681,100,000			596,000,000.0
Financing Receivable, Modified, Weighted-Average Term Increase from Modification			24 months
Repossessed Assets		0			0
Commitments to Fund Commercial Mortgage Loans		25,500,000
Real Estate Investment Property, Net		62,700,000			41,700,000
Real Estate Held-for-sale		$ 1,600,000			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%
Gross Securities Loaned		$ 100,800,000			106,100,000
Securities Received as Collateral		37,500,000			34,100,000
Cash Collateral for Borrowed Securities		67,600,000			76,100,000
Outstanding Repurchase Agreements		$ 0			0
Financing Receivable, Modified in Period, to Total Financing Receivables, Percentage		0.70%
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	[1]	$ 2,087,700,000			2,125,400,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	16,000,000.0	$ 15,600,000	$ 15,300,000	15,900,000	$ 16,600,000	$ 16,100,000
Accrued Investment Income		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			0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments		100,000			0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,425,100,000			$ 1,456,300,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		$ 92,400,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		921
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		744
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position		55
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		24

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