Parent Company Financial Information (Statements of Income) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income:
Other income									$ 5,521	$ 4,219	$ 5,260
Expenses:
Total interest expense	$ 5,297	$ 5,201	$ 4,916	$ 4,699	$ 4,748	$ 4,730	$ 4,682	$ 4,658	20,113	18,818	19,433
Income before income taxes	19,174	16,170	15,347	16,691	13,234	16,570	9,704	16,274	67,382	55,782	66,985
Income Tax Expense (Benefit)	(6,372)	(5,175)	(5,014)	(5,466)	(4,086)	(5,428)	(2,722)	(5,346)	(22,027)	(17,582)	(22,563)
Net income	$ 12,802	$ 10,995	$ 10,333	$ 11,225	$ 9,148	$ 11,142	$ 6,982	$ 10,928	45,355	38,200	44,422
Parent Company [Member]
Income:
Cash dividends from subsidiary									42,580	19,530	16,585
Other income									995	902	931
Total income									43,575	20,432	17,516
Expenses:
Interest									899	881	895
Other expenses									1,123	1,060	1,044
Total interest expense									2,022	1,941	1,939
Income before income taxes									41,553	18,491	15,577
Income Tax Expense (Benefit)									(308)	(266)	(271)
Income before equity in undistributed income of subsidiary									41,861	18,757	15,848
Equity in undistributed income of subsidiary									3,494	19,443	28,574
Net income									$ 45,355	$ 38,200	$ 44,422