Investments - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) Integer	Sep. 30, 2019 USD ($) Integer	Sep. 30, 2020 USD ($) Integer	Sep. 30, 2019 USD ($) Integer	Dec. 31, 2019 USD ($)
Schedule of Investments
Commitment to Fund Private Placement Fixed Maturity Securities	$ 185,300,000		$ 185,300,000
Other Long-term Investments	931,000,000.0		931,000,000.0		$ 844,200,000
Unfunded Commitments	1,800,000		1,800,000
Accrued Investment Income	676,900,000		676,900,000		693,000,000.0
Loan Foreclosure Expense	0	$ 0	0	$ 0
Commitments to Fund Commercial Mortgage Loans	$ 26,900,000		$ 26,900,000
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	$ 0		$ 0		176,400,000
Outstanding Repurchase Agreements	0		0		0
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral					186,500,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,421,100,000		2,421,100,000		2,397,000,000.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	14,600,000		14,600,000
Accrued Investment Income	$ 8,100,000		$ 8,100,000		8,300,000
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	$ 800,000
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments	200,000		200,000
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	725,200,000		725,200,000		675,100,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	36,600,000		36,600,000		58,400,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	$ 688,600,000		$ 688,600,000		$ 616,700,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	87		87
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	4		4
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	61		61
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	15		15