Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows From Operating Activities: [Abstract]
Net income	$ 446,424	$ 335,685	$ 232,281
Adjustments to reconcile net income to net cash provided by (used in) operating activities: [Abstract]
Provision for credit losses	8,000	(7,000)	65,000
Change in fair value of mortgage servicing rights due to market changes	(4,668)	(172)	2,193
Change in fair value of mortgage servicing rights due to principal payments	33,528	33,527	40,744
Net unrealized losses from derivative contracts	4,686	3,704	11,234
Share-based compensation	4,229	23,602	10,471
Depreciation and amortization	60,843	54,466	47,016
Net amortization of securities discounts and premiums	30,945	28,693	41,643
Net losses (gains) on financial instruments and other losses(gains), net	9,585	(2,828)	(13,011)
Net gain on mortgage loans held for sale	(35,705)	(47,159)	(63,636)
Mortgage loans originated for sale	(2,587,297)	(3,286,873)	(6,117,417)
Proceeds from sale of mortgage loans held for sale	2,691,144	3,405,890	6,193,587
Capitalized mortgage servicing rights	(35,247)	(39,149)	(71,405)
Change in trading and fair value option securities	(1,023,097)	(804,204)	149,921
Change in receivables	(38,346)	321,880	(603,861)
Change in other assets	27,507	(5,506)	(49,565)
Change in accrued interest, taxes and expense	(5,191)	18,191	44,269
Change in other liabilities	(139,346)	182,184	(11,413)
Net cash provided by (used in) operating activities	(552,006)	214,931	(91,949)
Cash Flows From Investing Activities: [Abstract]
Proceeds from maturities or redemptions of investment securities	124,864	112,022	86,847
Proceeds from maturities or redemptions of available for sale securities	1,122,680	1,841,217	1,740,226
Purchases of investment securities	(4,468)	(32,972)	(41,590)
Purchases of available for sale securities	(1,955,172)	(2,845,557)	(2,333,740)
Proceeds from sales of available for sale securities	745,643	1,309,215	899,381
Change in amount receivable on unsettled available for sale securities transactions	38,347	(68,792)	33,005
Loans originated, net of principal collected	(1,553,033)	(78,232)	(621,605)
Net payments on derivative asset contracts	(114,417)	479,409	(103,668)
Acquisitions, net of cash acquired	(175,755)	0	56,017
Proceeds from disposition of assets	308,762	274,029	198,922
Purchases of assets	(345,082)	(250,783)	(199,802)
Net cash provided by (used in) investing activities	(1,807,631)	739,556	(286,007)
Cash Flows From Financing Activities: [Abstract]
Net change in demand deposits, transaction deposits and savings accounts	(13,870)	(563,406)	1,277,285
Net change in time deposits	(73,089)	(123,384)	(216,084)
Net change in other borrowed funds	1,295,484	(10,909)	(606,476)
Repayment of subordinated debentures	0	0	(226,550)
Issuance of subordinated debentures, net of issuance costs	0	0	144,615
Change in amount due on unsettled security purchases	(41,319)	144,690	(10,389)
Issuance of common and treasury stock, net	(88)	4,368	12,455
Net change in derivative margin accounts	85,466	(17,726)	(28,806)
Net payments or proceeds on derivative liability contracts	114,076	(485,119)	106,051
Repurchase of common stock	(53,465)	(7,403)	(66,792)
Dividends paid	(127,188)	(116,041)	(113,455)
Net cash provided by (used in) financing activities	1,186,007	(1,174,930)	271,854
Net increase (decrease) in cash and cash equivalents	(1,173,630)	(220,443)	(106,102)
Cash and cash equivalents at beginning of period	2,317,054	2,537,497	2,643,599
Cash and cash equivalents at end of period	1,143,424	2,317,054	2,537,497
Supplemental Cash Flow Information: [Abstract]
Cash paid for interest	243,121	127,513	82,876
Cash paid for taxes	92,291	121,697	79,883
Net loans and bank premises transferred to repossessed real estate and other assets	9,880	7,367	36,391
Increase in U.S. government guaranteed loans eligible for repurchase	100,238	148,107	120,406
Increase in receivables from conveyance of GNMA OREO	$ 38,216	$ 40,528	$ 68,873