Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2022 USD ($) Integer	Sep. 30, 2021 USD ($) Integer	Sep. 30, 2022 USD ($) Integer	Sep. 30, 2021 USD ($) Integer	Dec. 31, 2021 USD ($)
Schedule of Investments
Other Long-term Investments	$ 1,430,400,000		$ 1,430,400,000		$ 1,203,000,000
Unfunded Commitments	700,000		700,000
Accrued Investment Income	717,200,000		717,200,000		602,700,000
Loan Foreclosure Expense	0	$ 0	0	$ 0
Commitments to Fund Commercial Mortgage Loans	9,000,000		9,000,000
Real Estate Held-for-sale	$ 44,300,000		$ 44,300,000		45,000,000.0
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	$ 231,000,000		$ 231,000,000		283,700,000
Gross Securities Borrowed	100,100,000		100,100,000		94,800,000
Outstanding Repurchase Agreements	0		0		0
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral	139,100,000		139,100,000		198,600,000
Cash Collateral for Borrowed Securities	100,100,000		100,100,000		94,800,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,485,100,000		2,485,100,000		2,568,700,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	8,700,000		8,700,000		8,300,000
Accrued Investment Income	$ 7,700,000		$ 7,700,000		8,100,000
Financing Receivable, Modifications, Number of Contracts | Integer	0	0	0	0
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		100,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,127,600,000		1,127,600,000		987,900,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	3,000,000		3,000,000		9,300,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,124,600,000		1,124,600,000		$ 978,600,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	$ 30,500,000		$ 30,500,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	892		892
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	114		114
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	118		118
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	11		11