Condensed Parent Company Financial Statements- Summary of Income Statement of Parent Company (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
OPERATING INCOME
Interest on interest-bearing deposits									$ 4,391	$ 4,064	$ 4,201
OPERATING EXPENSE
Other									36,407	32,823	32,832
Income before taxes									91,171	80,842	79,004
Allocated income tax benefit									(27,284)	(26,525)	(27,104)
Amortization of stock-based compensation arrangements of subsidiaries									(1,676)	(1,360)	(1,530)
Net income	15,749	18,793	14,688	14,657	13,861	14,491	12,593	13,372	63,887	54,317	51,900
Parent Company [Member]
OPERATING INCOME
Dividends from subsidiaries									24,851	22,612	20,784
Interest on interest-bearing deposits									62	47	57
Other									14	92
Total operating income									24,927	22,751	20,841
OPERATING EXPENSE
Interest									1,965	1,965	2,134
Other									667	661	942
Total operating expense									2,632	2,626	3,076
Income before taxes									22,295	20,125	17,765
Allocated income tax benefit									472	978	1,371
Income before equity in undistributed earnings of subsidiaries									22,767	21,103	19,136
Equity in undistributed earnings of subsidiaries									42,196	34,351	34,281
Amortization of stock-based compensation arrangements of subsidiaries									(1,076)	(1,137)	(1,517)
Net income									$ 63,887	$ 54,317	$ 51,900