Investments - Additional Information (Detail)	3 Months Ended			9 Months Ended
	Sep. 30, 2024 USD ($) Integer	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($) Integer	Sep. 30, 2024 USD ($) Integer	Sep. 30, 2023 USD ($) Integer	Dec. 31, 2023 USD ($)
Schedule of Investments
Other Long-term Investments	$ 1,682,800,000			$ 1,682,800,000		$ 1,579,400,000
Accrued Investment Income	$ 786,800,000			786,800,000		633,900,000
Write-offs		$ 0		$ 0
Financing Receivable, Modified, Weighted Average Term Increase from Modification	18 months			18 months
Gains (Losses) on Sales on Loan Foreclosure	$ 0		$ 0	$ 0	$ 0
Commitments to Fund Commercial Mortgage Loans	28,500,000			28,500,000
Real Estate Investment Property, Net	60,800,000			60,800,000		64,400,000
Real Estate Held-for-sale	$ 40,900,000			$ 40,900,000		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%			102.00%
Gross Securities Loaned	$ 57,900,000			$ 57,900,000		72,000,000.0
Outstanding Repurchase Agreements	0			0		0
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral	34,400,000			34,400,000		12,500,000
Cash Collateral for Borrowed Securities	25,800,000			25,800,000		63,100,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,255,400,000			2,255,400,000		2,328,400,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	16,500,000			16,500,000		10,200,000
Accrued Investment Income	7,000,000.0			7,000,000.0		7,200,000
Financing Receivable, Modified in Period, Amount	14,200,000			14,200,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0			0		0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	$ 0		$ 0	$ 0	$ 0
Financing Receivable, Modifications, Number of Contracts | Integer	0		0	0	0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments	$ 100,000			$ 100,000		0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,428,300,000			1,428,300,000		1,326,500,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	38,000,000.0			38,000,000.0
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	300,000			300,000		300,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 1,428,000,000			$ 1,428,000,000		$ 1,326,200,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	835			835
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	795			795
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	58			58
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	50			50