Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING ACTIVITIES
Net income	$ 254,904	$ 238,167	$ 215,713
Items not requiring (providing) cash
Stock-based compensation expense	13,197	12,921	9,725
Deferred income taxes	(122)	34,911	8,412
Changes in:
Other assets	(7,480)	31,920	(35,113)
Net cash provided by operating activities	202,543	256,078	226,980
INVESTING ACTIVITIES
Net collections (originations) of loans	1,327,248	23,806	(912,793)
Proceeds from maturities of available-for-sale securities	2,346,930	793,270	258,182
Purchases of available-for-sale securities	(4,140,963)	(1,708,948)	(784,113)
Net cash provided by (used in) investing activities	1,189,869	604,477	(1,395,122)
FINANCING ACTIVITIES
Proceeds from issuance of subordinated notes and other borrowings	0	25,500	326,355
Stock repurchases	113,327	10,128	0
Dividends paid on preferred stock	(52)	(339)	0
Dividends paid on common stock	(74,593)	(63,921)	(55,646)
Retirement of preferred stock	0	(42,000)	0
Net cash provided by (used in) financing activities	1,083,117	(697,390)	1,403,558
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	996,623	833,458	598,042
CASH AND CASH EQUIVALENTS, END OF YEAR	3,472,152	996,623	833,458
Parent Company
OPERATING ACTIVITIES
Net income	254,904	238,167	215,713
Items not requiring (providing) cash
Stock-based compensation expense	13,197	12,921	9,725
Depreciation and amortization	1,796	1,637	880
Deferred income taxes	1,583	(3,632)	26
Equity in undistributed net income of bank subsidiaries	29,345	(191,530)	(91,057)
Changes in:
Other assets	(27,056)	3,299	1,524
Other liabilities	7,790	(2,648)	17,340
Net cash provided by operating activities	281,559	58,214	154,151
INVESTING ACTIVITIES
Net collections (originations) of loans	186	117	219
Net (purchases of) proceeds from premises and equipment	(7)	(23,184)	3,342
(Advances to) repayment for subsidiaries	(15,363)	0	2,667
Proceeds from maturities of available-for-sale securities	0	2,544	152
Purchases of available-for-sale securities	0	(439)	(211)
Cash paid in business combinations	0	(36,811)	0
Other, net	185	29	(1,903)
Net cash provided by (used in) investing activities	(14,999)	(57,744)	4,266
FINANCING ACTIVITIES
Proceeds from issuance of subordinated notes and other borrowings	0	0	326,355
(Repayment ) issuance of long-term debt, net	(7,442)	2,000	(231,352)
(Cancellation) issuance of common stock, net	(3,131)	(1,077)	2,188
Stock repurchases	113,327	10,128	0
Dividends paid on preferred stock	(52)	(339)	0
Dividends paid on common stock	(74,593)	(63,921)	(55,646)
Retirement of preferred stock	0	(42,000)	0
Net cash provided by (used in) financing activities	(198,545)	(115,465)	41,545
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	68,015	(114,995)	199,962
CASH AND CASH EQUIVALENTS, BEGINNING OF YEAR	104,068	219,063	19,101
CASH AND CASH EQUIVALENTS, END OF YEAR	$ 172,083	$ 104,068	$ 219,063