Employee Benefit Plans - Schedule of Vesting Activity Relating to Restricted Stock Units Outstanding (Detail) (Restricted Stock Units (RSUs) [Member], USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
BALANCE, RSUs VESTED	66,800	45,400	29,800
Vested, NUMBER OF RSUs VESTED	29,205	21,400	15,600
Settled, NUMBER OF RSUs VESTED	(2,780)
BALANCE, RSUs VESTED	93,225	66,800	45,400
Vested, FAIR VALUE	$ 734,000	$ 505,000	$ 379,000
Settled, FAIR VALUE	$ 70,000