Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Reinsurance collateral	$ 3,200.0	$ 3,200.0
Reinsurance recoveries	2,700.0	2,800.0	$ 3,100.0
Berkshire Hathaway Group [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	2.0
Palo Verde Insurance Company [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	289.0
Wilton Re [Member]
Effects of Reinsurance [Line Items]
Largest recoverables from a single reinsurer	226.0
Significant Captive Program [Member]
Effects of Reinsurance [Line Items]
Direct and ceded earned premiums	$ 3,600.0	3,700.0	3,900.0
Percentage reinsured of direct and ceded earned premiums	100.00%
Reinsurance recoveries	$ 2,100.0	$ 1,900.0	$ 2,500.0