Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is pre-tax.

No definition available.

Investments - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
Investments - Additional Information (Detail) - USD ($)	Apr. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Principal amount of bonds and certificates of deposit on deposit to meet state insurance regulatory and/or rating agency requirements		$ 281,000,000.0
Securities exceeding 10% of Shareholders' Equity		0	$ 0
Fair value of fixed-maturity securities that were non-income producing during the preceding 12 months		0
Securities Sold under Agreements to Repurchase		0	0
Open reverse repurchase commitments		0	0
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Pre-Tax, Available-for-sale, Debt Securities		19,600,000	19,800,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held		500,000	500,000	$ 500,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)		$ 0	$ 0
Investment Portfolio Duration		3 years	2 years 9 months 18 days	2 years 6 months
Derivative Assets (Liabilities), at Fair Value, Net		$ 0	$ 0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax		(1,000,000.0)	(1,000,000.0)	$ 300,000
4.125% Senior Notes due 2047
Schedule of Investments [Line Items]
Senior notes, issuance amount	$ 850,000,000	$ 850,000,000	850,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.125%	4.125%
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	$ 8,000,000.0
Fixed maturities
Schedule of Investments [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held				500,000	$ 11,500,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment		$ 0		400,000
Non-hedging Instruments \| Interest Rate Swaps
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net		$ 0	$ 0	$ 0