Condensed Financial Information - Parent Company - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net earnings	$ 150,638	$ 120,371	$ 104,774
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization, net	12,549	12,916	11,573
Decrease (increase) in other assets	(1,697)	9,313	2,397
Increase (decrease) in other liabilities	1,621	285	(2,643)
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash received in connection with acquisition of banks	18,653
Maturity of available-for-sale security	3,439,028	4,392,131	3,509,113
Purchases of bank premises and equipment and software	(17,646)	(14,162)	(20,399)
Cash flows from financing activities:
Proceeds of stock issuances	3,864	2,934	1,260
Cash dividends paid	(53,861)	(48,955)	(44,907)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(123,700)	115,861	(16,400)
CASH AND CASH EQUIVALENTS, beginning of year	372,347	256,486	272,886
CASH AND CASH EQUIVALENTS, end of year	248,647	372,347	256,486
Parent Company [Member]
Cash flows from operating activities:
Net earnings	150,638	120,371	104,774
Adjustments to reconcile net earnings to net cash provided by operating activities:
Excess of earnings over dividends of subsidiary bank	(82,323)	(92,929)	(58,809)
Depreciation and amortization, net	331	207	208
Decrease (increase) in other assets	560	438	1,702
Increase (decrease) in other liabilities	1,932	183	(1,374)
Other	(2)	2	8
Net cash provided by operating activities	71,136	28,272	46,509
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash received in connection with acquisition of banks	0	0	0
Maturity of available-for-sale security	2,000	2,997
Purchases of bank premises and equipment and software	(346)	(30)	(94)
Other			10
Net cash provided by (used in) investing activities	1,654	2,967	(84)
Cash flows from financing activities:
Proceeds of stock issuances	3,864	2,934	1,260
Cash dividends paid	(53,861)	(48,955)	(44,907)
Net cash used in financing activities	(49,997)	(46,021)	(43,647)
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	22,793	(14,782)	2,778
CASH AND CASH EQUIVALENTS, beginning of year	78,469	93,251	90,473
CASH AND CASH EQUIVALENTS, end of year	$ 101,262	$ 78,469	$ 93,251

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