Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2021 USD ($) Integer	Dec. 31, 2020 USD ($) Integer	Dec. 31, 2019 USD ($)
Schedule of Investments
Commitment to Fund Private Placement Fixed Maturity Securities	$ 109,500,000
Other Long-term Investments	1,203,000,000	$ 960,200,000
Unfunded Commitments	700,000
Financing Receivable, Allowance for Credit Losses	8,300,000	13,100,000
Accrued Investment Income	602,700,000	611,400,000
Foreclosed Real Estate Expense	0	0	$ 0
Average Recorded Investment of Impaired Mortgage Loans			600,000
Committments to Fund Commercial Mortgage Loans	26,300,000	11,400,000
Real Estate Investment Property, Net	119,500,000	106,300,000
Real Estate Investment Property, Accumulated Depreciation	171,300,000	97,700,000
Impairment of Real Estate	$ 0	36,600,000	$ 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%
Gross Securities Borrowed	$ 283,700,000	96,600,000
Outstanding Repurchase Agreements	0	0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	94,800,000	17,600,000
Securities Received as Collateral	198,600,000	82,800,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss	2,568,700,000	2,445,200,000
Financing Receivable, Allowance for Credit Losses	8,300,000	13,100,000
Accrued Investment Income	$ 8,100,000	$ 8,000,000
Financing Receivable, Modifications, Number of Contracts | Integer	0	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	0	$ 0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments	100,000	100,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	987,900,000	776,800,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	9,300,000	29,300,000
Private Equity Partnerships | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 978,600,000	747,500,000
Investment Real Estate
Schedule of Investments
Impairment of Real Estate		$ 36,600,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	11
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	204
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	8
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	22