Condensed Parent Company Financial Statements - Summary of Cash Flow of Parent Company (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 18,620	$ 17,982	$ 17,493	$ 16,579	$ 15,728	$ 15,630	$ 18,553	$ 16,259	$ 70,674	$ 66,170	$ 63,887
Adjustments to reconcile to net cash provided by operating activities:
Amortization of stock-based compensation arrangements									1,636	1,717	1,676
Other, net									(5,500)	6,301	5,719
Net cash provided by operating activities									90,541	78,159	76,922
INVESTING ACTIVITIES
Net cash and due from banks (paid for) received from acquisitions										(4,007)	174,283
Net cash used in investing activities									(98,017)	(265,298)	(183,376)
FINANCING ACTIVITIES
Issuance of common stock, net									13,354	4,428	6,532
Common stock acquired									(5,523)	(1,654)
Cash dividends paid									(22,770)	(21,449)	(19,543)
Net cash provided by (used in) financing activities									259,760	(128,579)	162,814
Net increase in cash									252,284	(315,718)	56,360
Cash, due from banks and interest-bearing deposits at the beginning of the period				1,598,177				1,913,895	1,598,177	1,913,895	1,857,535
Cash, due from banks and interest-bearing deposits at the end of the period	1,850,461				1,598,177				1,850,461	1,598,177	1,913,895
SUPPLEMENTAL DISCLOSURE
Cash paid during the period for interest									14,673	12,255	13,135
Cash received during the period for income taxes, net									32,700	33,531	26,178
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
Net income									70,674	66,170	63,887
Adjustments to reconcile to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries									(44,172)	(44,653)	(42,196)
Amortization of stock-based compensation arrangements									401	1,172	1,076
Other, net									136	(343)	(656)
Net cash provided by operating activities									27,039	22,346	22,111
INVESTING ACTIVITIES
Net cash and due from banks (paid for) received from acquisitions										5,066
Sales and maturities of held for investment and available for sale securities											8
Net cash used in investing activities										5,066	8
FINANCING ACTIVITIES
Issuance of common stock, net									13,354	4,428	6,532
Common stock acquired									(5,523)	(1,654)
Cash dividends paid									(22,770)	(21,449)	(19,543)
Net cash provided by (used in) financing activities									(14,939)	(18,675)	(13,011)
Net increase in cash									12,100	8,737	9,108
Cash, due from banks and interest-bearing deposits at the beginning of the period				$ 45,437				$ 36,700	45,437	36,700	27,592
Cash, due from banks and interest-bearing deposits at the end of the period	$ 57,537				$ 45,437				57,537	45,437	36,700
SUPPLEMENTAL DISCLOSURE
Cash paid during the period for interest									2,096	1,998	1,965
Cash received during the period for income taxes, net									$ 5,031	$ 1,131	$ 1,822