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:
Net income	$ 422,417	$ 466,201	$ 495,401
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	18,256	37,242	4,180
Depreciation	32,323	32,803	32,811
Amortization of discounts and premiums, net	(3,891)	(4,555)	(26,258)
Amortization of core deposit intangibles		208	2,391
Net loss (gain) on sales of securities	14	(29,924)	(3,347)
Gain on trading securities activity	(222)	(316)
Net gain on sales of loans	(111)	(87,301)	(57,398)
Stock-based compensation	36,260	36,029	32,661
Deferred tax expense	23,197	21,444	44,746
Changes in operating assets and liabilities:
Decrease in other assets	29,952	451,873	326,790
(Decrease) increase in other liabilities	(53,320)	23,329	(4,336)
Purchases of securities held for trading	(141,615)	(202,450)
Proceeds from sales of securities held for trading	141,837	202,766
Origination of loans held for sale		(1,674,123)	(4,646,773)
Proceeds from sales of loans originated for sale	35,258	2,053,484	4,554,785
Net cash provided by operating activities	540,355	1,326,710	755,653
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from repayment of securities held to maturity		175,375	2,499,205
Proceeds from repayment of securities available for sale	817,822	387,772	50,192
Proceeds from sales of securities held to maturity		547,925	1,297
Proceeds from sales of securities available for sale	278,539	453,878	322,038
Purchase of securities held to maturity		(13,030)	(213,208)
Purchase of securities available for sale	(3,288,204)	(1,163,043)	(279,402)
Redemption of Federal Home Loan Bank stock	120,220	90,909	601,941
Purchases of Federal Home Loan Bank stock	(160,991)	(103,794)	(528,904)
Proceeds from bank-owned life insurance	16,303
Proceeds from sales of loans	195,760	2,289,377	1,675,550
Other changes in loans, net	(1,990,068)	(1,575,846)	(2,826,365)
Purchase of premises and equipment, net	(9,847)	(27,783)	(84,179)
Net cash (used in) provided by investing activities	(4,020,466)	1,061,740	1,218,165
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase in deposits	1,662,267	214,260	461,145
Net decrease in short-term borrowed funds		(460,000)	(3,256,300)
Proceeds from issuance of long-term borrowed funds	5,667,268	3,000,000	1,181,000
Repayments of long-term borrowed funds	(4,373,500)	(3,300,000)	0
Net proceeds from issuance of preferred stock		502,840
Cash dividends paid on common stock	(333,061)	(332,147)	(330,810)
Cash dividends paid on preferred stock	(32,828)	(24,621)
Treasury stock repurchased	(160,767)
Payments relating to treasury shares received for restricted stock award tax payments	(2,482)	(18,463)	(8,677)
Net cash provided by (used in) financing activities	2,426,897	(418,131)	(1,953,642)
Net (decrease) increase in cash and cash equivalents	(1,053,214)	1,970,319	20,176
Cash and cash equivalents at beginning of year	2,528,169	557,850	537,674
Cash and cash equivalents at end of year	1,474,955	2,528,169	557,850
Supplemental information:
Cash paid for interest	645,588	447,476	382,135
Cash paid for income taxes	44,123	217,682	180,238
Non-cash investing and financing activities:
Transfers to repossessed assets from loans	5,631	9,973	20,099
Transfer of loans from held for investment to held for sale	195,649	1,910,121	1,659,743
Shares issued for restricted stock awards	8,879	11,028	$ 8,985
Securities transferred from held to maturity to available for sale		$ 3,040,305