Income Taxes - Summary of Differences in Taxes Computed at Statutory Rate (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Tax Disclosure [Abstract]
Provision at statutory rate of 35%	$ 14,774	$ 13,333	$ 26,792	$ 23,855
Tax-exempt income	(3,092)	(2,636)	(5,989)	(5,146)
State income taxes	1,156	1,133	2,372	2,027
State statutory rate change	1,257		1,257
Interim period effective rate adjustment	(358)	(723)	(223)	(706)
Other, net	(3)	(218)	(83)	(801)
Income tax expense	$ 13,734	$ 10,889	$ 24,126	$ 19,229
Effective tax rate	32.50%	28.60%	31.50%	28.20%