Investments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2021 USD ($) Integer	Mar. 31, 2020 USD ($) Integer	Dec. 31, 2020 USD ($)
Schedule of Investments
Commitment to Fund Private Placement Fixed Maturity Securities	$ 106,500,000
Other Long-term Investments	995,300,000		$ 960,200,000
Unfunded Commitments	900,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	11,500,000		13,100,000
Accrued Investment Income	668,700,000		611,400,000
Loan Foreclosure Expense	0	$ 0
Commitments to Fund Commercial Mortgage Loans	$ 18,000,000.0
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%
Gross Securities Borrowed	$ 220,000,000.0		96,600,000
Outstanding Repurchase Agreements	0		0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	43,500,000		17,600,000
Securities Received as Collateral	186,400,000		82,800,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,413,700,000		2,445,200,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	11,500,000
Accrued Investment Income	$ 8,000,000.0		8,000,000.0
Financing Receivable, Modifications, Number of Contracts | Integer	0	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 0		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
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	816,800,000		776,800,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	24,400,000		29,300,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 792,400,000		$ 747,500,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	239
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	11
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	12