Treasury Stock - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended		84 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Equity Class Of Treasury Stock [Line Items]
Common stock amount authorized under repurchase program, value	$ 94.7		$ 94.7
Stock repurchased during period, shares	773,635	906,285	5,989,849
Average stock price of shares repurchased	$ 16.52	$ 13.23	$ 9.23
Maximum
Equity Class Of Treasury Stock [Line Items]
Common stock amount authorized under repurchase program, value	$ 150.0		$ 150.0