Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) Integer	Jun. 30, 2020 USD ($) Integer	Jun. 30, 2021 USD ($) Integer	Jun. 30, 2020 USD ($) Integer	Dec. 31, 2020 USD ($)
Schedule of Investments
Commitment to Fund Private Placement Fixed Maturity Securities	$ 148,600,000		$ 148,600,000
Other Long-term Investments	1,062,900,000		1,062,900,000		$ 960,200,000
Unfunded Commitments	700,000		700,000
Accrued Investment Income	722,200,000		722,200,000		611,400,000
Loan Foreclosure Expense	0	$ 0	0	$ 0
Commitments to Fund Commercial Mortgage Loans	$ 58,500,000		$ 58,500,000
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 Borrowed	$ 231,700,000		$ 231,700,000		96,600,000
Outstanding Repurchase Agreements	0		0		0
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral	130,800,000		130,800,000		82,800,000
Cash Collateral for Borrowed Securities	108,500,000		108,500,000		17,600,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,398,500,000		2,398,500,000		2,445,200,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	11,400,000		11,400,000		13,100,000
Accrued Investment Income	$ 7,800,000		$ 7,800,000		8,000,000.0
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
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments	200,000		200,000		100,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	877,600,000		877,600,000		776,800,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	19,400,000		19,400,000		29,300,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 858,200,000		$ 858,200,000		$ 747,500,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	131		131
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	6		6
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	15		15
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	8		8