Investments - Additional Information (Detail)		12 Months Ended
		Dec. 31, 2023 USD ($) Integer	Dec. 31, 2022 USD ($) Integer	Dec. 31, 2021 USD ($) Integer
Schedule of Investments
Financing Receivable, Troubled Debt Restructuring, Postmodification		$ 8,000,000
Other Long-term Investments		1,579,400,000	$ 1,440,100,000
Unfunded Commitments		200,000
Financing Receivable, Allowance for Credit Losses		10,200,000
Accrued Investment Income		633,900,000	615,000,000.0
Financing Receivable, Allowance for Credit Loss, Writeoff		$ 0	0
Financing Receivable, Modified, Weighted Average Term Increase from Modification		18 months
Foreclosed Real Estate Expense		$ 0	0	$ 0
Committments to Fund Commercial Mortgage Loans		0	5,000,000
Real Estate Investment Property, Net		64,400,000	71,600,000
Real Estate Held-for-sale		$ 40,900,000	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		$ 72,000,000.0	152,400,000
Outstanding Repurchase Agreements		0	0
Perpetual Preferred Securities
Schedule of Investments
Unrealized Gain (Loss) on Investments	[1]	0	2,800,000	4,400,000
Private Equity Partnerships
Schedule of Investments
Unrealized Gain (Loss) on Investments		102,900,000	124,100,000	177,700,000
Management Fee Expense		24,800,000	14,000,000.0	12,300,000
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities		63,100,000	88,500,000
Securities Received as Collateral		12,500,000	69,800,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, Troubled Debt Restructuring, Postmodification		14,200,000
Financing Receivable, before Allowance for Credit Loss		2,328,400,000	2,444,700,000
Financing Receivable, Allowance for Credit Losses		10,200,000	9,300,000	$ 8,300,000
Accrued Investment Income		7,200,000	7,700,000
Financing Receivable, Allowance for Credit Loss, Writeoff		$ 0	$ 0
Financing Receivable, Modifications, Number of Contracts | Integer		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
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments		0	0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,326,500,000	1,195,300,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		300,000	1,000,000
Private Equity Partnerships | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments		1,326,200,000	1,194,300,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability		$ 122,000,000
Public Utilities | Principal Forgiveness
Schedule of Investments
Financing Receivable, Troubled Debt Restructuring			4,800,000
Private Equity Funds
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		832
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		787
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions		75
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year		70

[1] The net unrealized gain (loss) recognized in net investment income for the year ended December 31, 2023 related to perpetual preferred securities still held at December 31, 2023 was nominal. The net unrealized gain (loss) recognized in net investment income for the years ended December 31, 2022 and 2021 related to perpetual preferred securities still held at year-end was $2.8 million and $4.4 million, respectively.