Components of income tax expense (benefit) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Disclosure Of Components Of Income Tax Expense Benefit [Line Items]
Federal	$ 11,199	$ 6,840	$ 8,106
State	2,913	2,327	1,423
Total current	14,112	9,167	9,529
Federal	3,116	5,640	(469)
State	817	1,038	(11)
Total deferred	3,933	6,678	(480)
Total income tax expense (benefit)	$ 18,045	$ 15,845	$ 9,049