Investments - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2023 USD ($) Integer	Jun. 30, 2022 USD ($) Integer	Jun. 30, 2023 USD ($) Integer	Jun. 30, 2022 USD ($) Integer	Dec. 31, 2022 USD ($)
Schedule of Investments
Other Long-term Investments	$ 1,503,200,000		$ 1,503,200,000		$ 1,440,100,000
Accrued Investment Income	715,100,000		715,100,000		615,000,000.0
Write-offs			0		0
Gains (Losses) on Sales on Loan Foreclosure	0	$ 0	0	$ 0
Commitments to Fund Commercial Mortgage Loans	0		0
Real Estate Investment Property, Net	70,300,000		70,300,000		71,600,000
Real Estate Held-for-sale	40,900,000		40,900,000		40,900,000
Real Estate, Reclassification to Held-for-Sale, Carrying Value	$ 40,100,000		$ 40,100,000		40,100,000
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 Loaned	$ 118,200,000		$ 118,200,000		152,400,000
Outstanding Repurchase Agreements	0		0		0
Equity Securities
Schedule of Investments
Unrealized Gain (Loss) on Investments	(200,000)	(1,400,000)	500,000	1,000,000
Private Equity Partnerships
Schedule of Investments
Unrealized Gain (Loss) on Investments	25,100,000	26,900,000	42,900,000	43,200,000
Management Fee Expense	(5,300,000)	$ (2,700,000)	(8,800,000)	$ (6,500,000)
Off Balance Sheet Amount
Schedule of Investments
Securities Received as Collateral	26,300,000		26,300,000		69,800,000
Cash Collateral for Borrowed Securities	97,500,000		97,500,000		88,500,000
Commercial Real Estate
Schedule of Investments
Financing Receivable, before Allowance for Credit Loss, Mortgage Loans	2,383,900,000		2,383,900,000		2,444,700,000
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	11,100,000		11,100,000		9,300,000
Accrued Investment Income	$ 7,400,000		$ 7,400,000		7,700,000
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	$ 0
Unfunded Mortgage Loan Commitments
Schedule of Investments
Allowance for Credit Losses, Unfunded Mortgage Loan Commitments	0		0		0
Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,268,800,000		1,268,800,000		1,195,300,000
Tax Credit Partnership | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	700,000		700,000		1,000,000
Equity Method Investments | Variable Interest Entity, Not Primary Beneficiary
Schedule of Investments
Other Long-term Investments	1,268,100,000		1,268,100,000		$ 1,194,300,000
Private Placement Fixed Maturity
Schedule of Investments
Fair Value Disclosure, off-Balance-Sheet Risks, Amount, Liability	$ 97,800,000		$ 97,800,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	878		878
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	714		714
External Credit Rating, Below-Investment-Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	106		106
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One Year	85		85