Investments - Additional Information (Detail)		12 Months Ended
		Dec. 31, 2022 USD ($) Integer	Dec. 31, 2021 USD ($) Integer	Dec. 31, 2020 USD ($) Integer
Schedule of Investments
Other Long-term Investments		$ 1,440,100,000	$ 1,203,000,000
Unfunded Commitments		700,000
Financing Receivable, Allowance for Credit Losses		9,300,000
Accrued Investment Income		615,000,000.0	602,700,000
Foreclosed Real Estate Expense		0	0	$ 0
Committments to Fund Commercial Mortgage Loans		5,000,000	26,300,000
Real Estate Investment Property, Net		71,600,000	119,500,000
Real Estate Investment Property, Accumulated Depreciation		122,100,000	171,300,000
Impairment of Real Estate		0	0	36,600,000
Real Estate Held-for-sale		$ 40,900,000
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%
Gross Securities Loaned		$ 152,400,000	283,700,000
Outstanding Repurchase Agreements		0	0
Financing Receivable, Troubled Debt Restructuring, Postmodification		8,000,000.0
Securities Loaned		152,400,000	283,700,000
Net Investment Gain (Loss)		(15,700,000)	76,700,000	1,199,100,000
Real Estate Investment
Schedule of Investments
Net Investment Gain (Loss)		0
Perpetual Preferred Securities
Schedule of Investments
Unrealized Gain (Loss) on Investments	[1]	2,800,000	4,400,000	$ (4,600,000)
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities		88,500,000	94,800,000
Securities Received as Collateral		69,800,000	198,600,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss		2,444,700,000	2,568,700,000
Financing Receivable, Allowance for Credit Losses		9,300,000	8,300,000
Accrued Investment Income		$ 7,700,000	$ 8,100,000
Financing Receivable, Modifications, Number of Contracts | Integer		0	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	$ 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		1,195,300,000	987,900,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,000,000	9,300,000
Private Equity Partnerships | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,194,300,000	978,600,000
Investment Real Estate
Schedule of Investments
Impairment of Real Estate		0	$ 0
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		58,700,000
Public Utilities | Principal Forgiveness
Schedule of Investments
Financing Receivable, Troubled Debt Restructuring		4,800,000
Private Equity Partnerships
Schedule of Investments
Financing Receivable, Troubled Debt Restructuring, Postmodification		$ 4,900,000
External Credit Rating, Investment Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions		882
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		262
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions		112
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		16

[1] The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2022 related to perpetual preferred securities still held at December 31, 2022 was $2.8 million. The net unrealized gain (loss) recognized in net investment income for the years ended December 31, 2021 and 2020 related to perpetual preferred securities still held at year-end was $4.4 million and $(4.6) million, respectively