Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Cash Flow From Operating Activities
Net income	$ 88,809	$ 94,301	$ 45,214	$ 45,838	$ 274,117	$ 239,769
Adjustments to reconcile net income to net cash provided by (used in) operating activities
Provision for credit losses	(24,010)		43,009		(82,018)	157,009
Depreciation and amortization					36,109	38,707
Addition to (recovery of) valuation allowance on mortgage servicing rights, net					(12,231)	18,481
Amortization of mortgage servicing rights					15,624	16,416
Amortization of other intangible assets	2,203		2,253		6,642	7,939
Amortization and accretion on earning assets, funding, and other, net					12,184	21,748
Net amortization of tax credit investments					25,196	18,988
Losses (gains) on sales of investment securities, net					16	(9,222)
Asset (gains) losses, net					(10,024)	(156,945)
(Gains) losses on sale of branch, net					(1,038)	0
(Gain) loss on mortgage banking activities, net					(32,304)	(43,889)
Mortgage loans originated and acquired for sale					(1,345,158)	(1,319,034)
Proceeds from sales of mortgage loans held for sale					1,348,006	1,620,777
Changes in certain assets and liabilities
(Increase) decrease in interest receivable					11,252	(415)
Increase (decrease) in interest payable					(13,287)	(12,735)
Increase (decrease) in expense payable					24,509	(32,892)
(Increase) decrease in net derivative position					82,122	(133,165)
(Increase) decrease in net income tax position					(19,940)	(58,002)
Net change in other assets and other liabilities					44,366	49,525
Net cash provided by (used in) operating activities					364,141	423,060
Cash Flow From Investing Activities
Net decrease (increase) in loans					804,497	(2,170,320)
Purchases of
AFS securities					(1,985,700)	(1,368,124)
HTM securities					(250,259)	(109,824)
Federal Home Loan Bank and Federal Reserve Bank stocks and equity securities					(9)	(84,152)
Premises, equipment, and software, net of disposals					(34,337)	(34,440)
Other intangibles					0	(200)
Proceeds from
Sales of securities					158,743	626,283
Sale of Federal Home Loan Bank and Federal Reserve Bank stocks					0	144,000
Prepayments, calls, and maturities of AFS investment securities					927,053	893,157
Prepayments, calls, and maturities of HTM investment securities					243,063	323,175
Sales, prepayments, calls, and maturities of other assets					18,149	18,457
Net change in tax credit and alternative investments					(45,655)	(35,630)
Net cash (paid) received in acquisition					0	31,518
Net cash provided by (used in) investing activities					(162,040)	(1,572,625)
Cash Flow From Financing Activities
Net increase (decrease) in deposits					1,400,162	2,495,229
Net decrease in deposits due to branch sales					(31,083)	0
Net increase (decrease) in short-term funding					70,179	(284,655)
Net increase (decrease) in short-term FHLB advances					0	(520,000)
Repayment of long-term FHLB advances					(18,276)	(966,777)
Proceeds from long-term FHLB advances					6,576	4,000
Proceeds from PPPLF					0	1,022,217
(Repayment) proceeds of finance lease principal					(1,056)	(1,044)
Repayments of Senior Debt					(300,000)	0
Proceeds from issuance of preferred shares			(208)		0	96,921
Proceeds from issuance of common stock for stock-based compensation plans					18,629	3,165
Redemption of preferred shares					(164,458)	0
Cash dividends on common stock					(86,238)	(84,156)
Cash dividends on preferred stock					(14,236)	(13,152)
Net cash provided by (used in) financing activities					767,695	1,674,484
Net increase (decrease) in cash and cash equivalents					969,796	524,918
Cash and cash equivalents at beginning of period		716,048		588,744	716,048	588,744	$ 588,744
Cash and cash equivalents at end of period(a)	1,685,843		1,113,663		1,685,843	1,113,663	716,048
Supplemental disclosures of cash flow information
Cash paid for interest					69,470	137,423
Cash paid for (received from) income and franchise taxes					56,262	17,682
Loans and bank premises transferred to OREO					33,794	12,599
Capitalized mortgage servicing rights					11,761	11,495
Loans transferred into held for sale from portfolio, net					10,071	260,856
Unsettled trades to purchase securities					9,855	1,000
Acquisition
Fair value of assets acquired, including cash and cash equivalents	0		457,878		0	457,878
Fair value ascribed to goodwill and intangible assets	0		22,150		0	22,150
Fair value of liabilities assumed	0		480,028		0	480,028
Restricted Cash and Cash Equivalents
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Total	1,685,843		1,113,663		1,685,843	1,113,663	$ 716,048
Retained Earnings
Cash Flow From Operating Activities
Net income	$ 88,809	$ 94,301	$ 45,214	$ 45,838	274,117	239,769
ABRC
Proceeds from
Sales, prepayments, calls, and maturities of other assets						256,511
Whitnell
Proceeds from
Sales, prepayments, calls, and maturities of other assets					2,415
Performance/Service Based RSAs, RSA,
Cash Flow From Financing Activities
Payments for Repurchase of Common Stock					(4,562)	(6,010)
Board Authorized Purchase Program [Member]
Cash Flow From Financing Activities
Payments for Repurchase of Common Stock					$ (107,943)	$ (71,255)