Stock Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Schedule Of Stock Option Activity [Line Items]
Number of shares, Beginning Balance	352,942
Number of shares, Granted
Number of shares, Exercised	(130,304)
Number of shares, Forfeited	(2,289)
Number of shares, Ending Balance	220,349
Vested and exercisable at December 31, 2012	195,380
Options expected to vest at December 31, 2012	21,985
Weighted Average Exercise Price, Beginning Balance	$ 20.21
Weighted Average Exercise Price, Granted
Weighted Average Exercise Price, Exercised	$ 20.23
Weighted Average Exercise Price, Forfeited	$ 37.01
Weighted Average Exercise Price, Ending Balance	$ 20.03
Vested and exercisable at December 31, 2012	$ 18.31
Options expected to vest at December 31, 2012	$ 33.49