Income Taxes - Summary of Differences in Taxes from Continuing Operations Computed at Statutory Rate (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Provision at statutory rate of 35%	$ 10,546	$ 12,513
Tax-exempt income	(3,852)	(3,137)
State income taxes	1,277	643
Interim period effective rate adjustment	1,506	(2,025)
State statutory rate change		1,122
Other, net	(252)	126
Income tax expense	$ 9,225	$ 9,242
Effective tax rate	30.60%	25.90%