Condensed Financial Information (Parent Company Only) - Condensed 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
Operating Income (Expense)
Other income									$ 1,103	$ 1,013	$ 1,103
Interest expense	$ (3,568)	$ (3,802)	$ (3,277)	$ (3,207)	$ (3,242)	$ (3,451)	$ (3,334)	$ (3,195)	(13,854)	(13,222)	(13,503)
Employee benefit expense									(37,712)	(32,682)	(31,032)
Income Before Income Tax									27,781	24,256	26,924
Income Tax Benefit									(7,232)	(6,155)	(7,048)
Net income	6,175	4,288	4,728	5,358	4,948	4,958	4,778	3,417	20,549	18,101	19,876
Net Income Available to Common Shareholders	6,144	4,257	4,697	5,327	4,917	4,918	4,747	3,386	20,424	17,968	19,506
Net income	$ 6,175	$ 4,288	$ 4,728	$ 5,358	$ 4,948	$ 4,958	$ 4,778	$ 3,417	20,549	18,101	19,876
Other Comprehensive Income (Loss)
Change in fair value of derivative instruments, net of taxes									127	(332)	1,734
Other Comprehensive Income (Loss), Net of Tax									(2,244)	3,250	(10,830)
Comprehensive Income									18,305	21,351	9,046
Parent Company [Member]
Operating Income (Expense)
Dividend income from Bank									30,470	12,500	7,500
Investment income									15	12	4
Other income									24	17	175
Interest expense									(2,009)	(2,009)	(2,010)
Employee benefit expense									(1,093)	(965)	(811)
Other expense									910	883	646
Income Before Undistributed Income of Subsidiaries									28,317	10,438	5,504
Undistributed Income of Subsidiaries									(8,168)	6,814	13,144
Income Before Income Tax									20,149	17,252	18,648
Income Tax Benefit									400	849	1,228
Net income									20,549	18,101	19,876
Preferred stock dividend									(125)	(133)	(370)
Net Income Available to Common Shareholders									20,424	17,968	19,506
Net income									20,549	18,101	19,876
Other Comprehensive Income (Loss)
Change in fair value of derivative instruments, net of taxes									127	(332)	1,734
Unrealized appreciation for the period on held-to-maturity securities, net of taxes									(357)	1,078
Unrealized appreciation (depreciation) on available-for-sale securities, net of taxes									(1,891)	3,146	(12,320)
Less: reclassification adjustment for realized gains included in net income, net of taxes									(123)	(642)	(244)
Other Comprehensive Income (Loss), Net of Tax									(2,244)	3,250	(10,830)
Comprehensive Income									$ 18,305	$ 21,351	$ 9,046