Condensed Parent Company Financial Statements - Schedule of Condensed Cash Flow Statement (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
OPERATING ACTIVITIES
Net income	$ 36,376	$ 37,347	$ 44,814	$ 40,337	$ 43,928	$ 32,486	$ 33,169	$ 33,529	$ 158,873	$ 143,112	$ 94,482
Adjustments to reconcile net income to net cash provided by operating activities:
Net change in other assets									(33,400)	19,895	4,593
Net securities (gains) losses									(4,320)	900	(567)
Other—net									(139)	(233)	(438)
Net cash provided by operating activities									163,363	191,891	142,080
INVESTING ACTIVITIES
Proceeds from sales—securities available-for-sale									125,839	82,134	7,760
Purchases of securities—securities available-for-sale									(573,729)	(841,696)	(252,114)
Acquisitions and additional capitalization of subsidiaries, net of cash acquired									60,025	278,654
Net cash provided by (used in) investing activities									103,786	(51,859)	(77,375)
FINANCING ACTIVITIES
Repayment of junior subordinated debt									(33,506)	(17,519)
Issuance of common stock									72	1,578	1,040
Treasury shares purchased—net									(10,211)	(426)
Dividends paid to common and preferred shareholders									(66,571)	(53,577)	(44,864)
Net cash used in financing activities									(201,539)	(88,418)	(75,303)
Net increase (decrease) in cash and cash equivalents									65,610	51,614	(10,598)
Cash and cash equivalents at beginning of the year				169,186				117,572	169,186	117,572	128,170
Cash and cash equivalents at end of the year	234,796				169,186				234,796	169,186	117,572
Parent Company [Member]
OPERATING ACTIVITIES
Net income									158,873	143,112	94,482
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed net income									(64,214)	(67,454)	(24,996)
Net change in other assets									(5,443)	(3,612)	566
Net securities (gains) losses									(19)	36
Other—net									6,898	4,988	2,848
Net cash provided by operating activities									96,095	77,070	72,900
INVESTING ACTIVITIES
Proceeds from sales—securities available-for-sale									1,007	1,511
Purchases of securities—securities available-for-sale											(200)
Acquisitions and additional capitalization of subsidiaries, net of cash acquired									62,112	37,309
Net cash provided by (used in) investing activities									63,119	38,820	(200)
FINANCING ACTIVITIES
Repayment of junior subordinated debt									(33,506)	(17,519)
Issuance of common stock									72	1,578	1,040
Treasury shares purchased—net									(10,211)	(426)
Dividends paid to common and preferred shareholders									(66,572)	(53,577)	(44,864)
Net cash used in financing activities									(110,217)	(69,944)	(43,824)
Net increase (decrease) in cash and cash equivalents									48,997	45,946	28,876
Cash and cash equivalents at beginning of the year				$ 121,857				$ 75,911	121,857	75,911	47,035
Cash and cash equivalents at end of the year	$ 170,854				$ 121,857				$ 170,854	$ 121,857	$ 75,911