Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) Integer	Dec. 31, 2019 USD ($) Integer	Dec. 31, 2018 USD ($) Integer
Schedule of Investments
Commitment to Fund Private Placement Fixed Maturity Securities	$ 21,000,000.0
Other Long-term Investments	960,200,000	$ 844,200,000
Unfunded Commitments	900,000
Financing Receivable, Allowance for Credit Losses	13,100,000	0
Accrued Investment Income	611,400,000	693,000,000.0
Foreclosed Real Estate Expense	0	0	$ 0
Average Recorded Investment of Impaired Mortgage Loans		600,000	2,300,000
Committments to Fund Commercial Mortgage Loans	11,400,000
Real Estate Investment Property, Net	106,300,000	90,500,000
Real Estate Investment Property, Accumulated Depreciation	97,700,000	49,800,000
Impairment of Real Estate	$ 36,600,000	0	$ 1,400,000
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 Borrowed	$ 96,600,000	176,400,000
Outstanding Repurchase Agreements	0	0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	17,600,000
Securities Received as Collateral	82,800,000	186,500,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss	2,445,200,000	2,397,000,000.0
Financing Receivable, Allowance for Credit Losses	13,100,000
Accrued Investment Income	$ 8,000,000.0	$ 8,300,000
Financing Receivable, Modifications, Number of Contracts | Integer	0	0	1,000,000
Financing Receivable, Modifications, Pre-Modification Recorded Investment			$ 3,600,000
Provision for Loan and Lease Losses		$ 100,000	200,000
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 0
Impaired Financing Receivable, with No Related Allowance, Average Recorded Investment	0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments	100,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	776,800,000	675,100,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	29,300,000	58,400,000
Private Equity Partnerships | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	747,500,000	$ 616,700,000
Investment Real Estate
Schedule of Investments
Impairment of Real Estate	$ 36,600,000		$ 600,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	47
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	5
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	21
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	13