Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OPERATING ACTIVITIES
Net income	$ 150,581	$ 147,083	$ 137,959
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	28,406	24,509	22,574
Depreciation, amortization and accretion	(21,567)	(8,720)	746
Loss (gain) on sales of bank premises, OREO and equipment	4,813	5,196	2,059
(Gain) loss on securities	(5,584)	(280)	(155)
Loans originated for sale	(2,373,622)	(154,335)	(145,836)
Proceeds from sales of loans	2,455,213	160,021	146,342
Gain on sales of loans	(58,907)	(3,450)	(2,507)
Stock-based compensation	3,555	2,817	2,484
Excess tax benefits from stock-based compensation arrangements	2,201	4,008	1,023
Deferred income tax expense	60,827	7,252	1,729
Increase in cash surrender value of bank-owned life insurance policies	(5,110)	(3,837)	(5,557)
Contribution to pension plan	(10,000)	0	0
Amortization of net periodic pension costs	3,310	5,156	4,380
Changes in:
Loans held for sale	(8,893)	0	0
Interest receivable	(2,024)	(755)	(3,467)
Other assets	32,924	(5,855)	14,349
Accrued expenses and other liabilities	(2,195)	(8,076)	(1,222)
Net Cash Provided by Operating Activities	253,928	170,734	174,901
INVESTING ACTIVITIES
Proceeds from maturities and calls of held to maturity securities	14,214	5,730	1,056
Purchases of held to maturity securities	(1,403)	0	(1,000)
Proceeds from sales of securities available for sale	247,131	103,440	7,316
Proceeds from maturities and calls of securities available for sale	439,181	410,550	183,950
Purchases of securities available for sale	(932,474)	(504,978)	(85,249)
Redemption of bank-owned life insurance policies	0	630	1,974
Purchases of bank premises and equipment	(14,357)	(7,271)	(5,263)
Proceeds from sales of bank premises and equipment	17	554	866
Acquisition of subsidiaries, net of cash paid	44,531	29,330	0
Proceeds from sales and redemptions of other investment securities	40,837	64,411	19,845
Purchases of other investment securities	(74,090)	(72,052)	(22,181)
Proceeds from sales of OREO properties	7,066	17,871	10,270
Net change in loans	496,025	(17,255)	(296,882)
NET CASH PROVIDED BY (USED IN) INVESTING ACTIVITIES	266,678	30,960	(185,298)
FINANCING ACTIVITIES
Cash dividends paid	(121,354)	(96,351)	(88,864)
Acquisition of treasury stock	(1)	(1)	(1)
Proceeds from exercise of stock options	4,619	13,337	7,871
Proceeds from the issuance of common stock	0	199,916	0
Distribution of treasury stock for deferred compensation plan	0	1	1
Repayment of long-term Federal Home Loan Bank borrowings	(845,208)	(725,673)	(794,455)
Proceeds of long-term Federal Home Loan Bank borrowings	815,000	795,000	705,000
Changes in:
Time deposits	614,821	75,639	(135,674)
Other deposits	(928,665)	408,127	433,414
Federal funds purchased, securities sold under agreements to repurchase and other short-term borrowings	171,821	(294,497)	(12,624)
Net Cash Provided by (Used in) Financing Activities	(288,967)	375,498	114,668
INCREASE IN CASH AND CASH EQUIVALENTS	231,640	577,192	104,271
Cash and Cash Equivalents at Beginning of Year	1,434,527	857,335	753,064
CASH AND CASH EQUIVALENTS AT END OF YEAR	1,666,167	1,434,527	857,335
Supplemental information
Cash paid for interest	72,715	44,609	39,835
Cash paid for income taxes	73,096	61,905	52,319
Noncash investing activities:
Transfers of loans to OREO	$ 17,615	$ 21,776	$ 5,857