Insurance Activity - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended
	Aug. 31, 2009	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 19, 2012
Effects of Reinsurance [Line Items]
Quoted percentage to share base	50.00%
Balances in loss contract reserve		$ 0.0	$ 5.6		$ 63.1
Decreased value of loss contract reserve		$ 5.6	$ 4.0	$ 2.9
WMMRC [Member]
Effects of Reinsurance [Line Items]
Second loss layer risk percentage of range minimum	5.00%
Second loss layer risk percentage of range maximum	10.00%
First loss layer risk percentage of range minimum	4.00%
First loss layer risk percentage of range maximum	5.00%
Minimum period of reinsurance agreements		5 years
Maximum period of reinsurance agreements		10 years
WMMRC [Member] | Minimum [Member] | Credit Concentration Risk | Liabilities, Total [Member]
Effects of Reinsurance [Line Items]
Net of ceding commission, percentage	25.00%
WMMRC [Member] | Maximum [Member] | Credit Concentration Risk | Liabilities, Total [Member]
Effects of Reinsurance [Line Items]
Net of ceding commission, percentage	40.00%