Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Investments
Other Long-term Investments	$ 1,694,400,000	$ 1,579,400,000
Unfunded Commitments	200,000
Financing Receivable, Allowance for Credit Losses	16,100,000	10,200,000
Accrued Investment Income	$ 649,800,000	633,900,000
Financing Receivable, Modified, Weighted Average Term Increase from Modification	18 months
Foreclosed Real Estate Expense	$ 0	0	$ 0
Committments to Fund Commercial Mortgage Loans	17,900,000	0
Real Estate Investment Property, Net	59,500,000	64,400,000
Real Estate Held-for-sale	$ 41,900,000	40,900,000
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	$ 94,000,000.0	72,000,000.0
Outstanding Repurchase Agreements	0	0
Financing Receivable, Nonaccrual	9,200,000	0	0
Financing Receivable, Nonaccrual, Interest Income	0	0	0
Private Equity Partnerships
Schedule of Investments
Unrealized Gain (Loss) on Investments	127,100,000	102,900,000	124,100,000
Management Fee Expense	24,000,000.0	24,800,000	14,000,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	62,700,000	63,100,000
Securities Received as Collateral	34,800,000	12,500,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss	2,240,600,000	2,328,400,000
Financing Receivable, Allowance for Credit Losses	16,100,000	10,200,000	$ 9,300,000
Accrued Investment Income	7,000,000.0	7,200,000
Financing Receivable, Allowance for Credit Loss, Writeoff	7,000,000.0	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	1,450,800,000	1,326,500,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	200,000	300,000
Private Equity Partnerships | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,450,600,000	$ 1,326,200,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	$ 96,500,000
External Credit Rating, Investment Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	924
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	785
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	75
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	47