Insurance Activity - Additional Information (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Mar. 19, 2012	Aug. 31, 2009
Quoted percentage to share base				50.00%
Balances in the fair market reserve	$ 51,200,000	$ 52,200,000
Loss contract fair market reserve	51,230,000	52,217,000	63,100,000
Decreased value of fair market reserve		10,900,000
Decrease in expense	$ 1,000,000
WMMRC [Member]
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