Investments - Additional Information (Detail)	3 Months Ended			9 Months Ended
	Sep. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Investments
Other Long-term Investments	$ 1,506,900,000			$ 1,506,900,000		$ 1,440,100,000
Accrued Investment Income	$ 704,900,000			704,900,000		615,000,000.0
Write-offs		$ 0		$ 0
Financing Receivable, Modified, Weighted Average Term Increase from Modification	18 months			18 months
Financing Receivable, Modified in Period, to Total Financing Receivables, Percentage	0.60%			0.60%
Gains (Losses) on Sales on Loan Foreclosure	$ 0		$ 0	$ 0	$ 0
Commitments to Fund Commercial Mortgage Loans	8,800,000			8,800,000
Real Estate Investment Property, Net	65,700,000			65,700,000		71,600,000
Real Estate Held-for-sale	$ 40,900,000			$ 40,900,000		40,900,000
Real Estate, Reclassification to Held-for-Sale, Carrying Value			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	$ 68,300,000			$ 68,300,000		152,400,000
Outstanding Repurchase Agreements	0			0		0
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral	12,300,000			12,300,000		69,800,000
Cash Collateral for Borrowed Securities	59,600,000			59,600,000		88,500,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,331,700,000			2,331,700,000		2,444,700,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	10,200,000			10,200,000		9,300,000
Accrued Investment Income	7,200,000			7,200,000		7,700,000
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
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	1,281,400,000			1,281,400,000		1,195,300,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	53,800,000			53,800,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	500,000			500,000		1,000,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 1,280,900,000			$ 1,280,900,000		$ 1,194,300,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	968			968
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	813			813
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	93			93
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	79			79