Consolidated Statements of Cash Flows (Unaudited) - USD ($)	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Operating activities
Net income	$ 29,992,000	$ 2,008,000	$ 37,446,000	$ 45,110,000	$ 52,130,000	$ 129,181,000
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for credit losses on loans	23,100,000		1,700,000		76,350,000	4,100,000
Depreciation, amortization and accretion					22,727,000	5,826,000
Deferred income tax (benefit) expense					(8,494,000)	13,911,000
Funding of mortgage loans held for sale					(3,277,576,000)	(1,680,729,000)
Proceeds from sales of mortgage loans held for sale					3,310,402,000	1,543,544,000
Gains on sales of mortgage loans held for sale					(114,327,000)	(35,416,000)
Valuation adjustment to mortgage servicing rights					13,694,000	3,132,000	$ 1,836,000
Gains on sales of securities	0		(343,000)		(31,000)	(348,000)
Penalty on prepayment of debt	28,000		54,000		118,000	54,000
Loss (gains) on sales of premises and equipment					35,000	(1,062,000)
Stock-based compensation expense					8,153,000	7,721,000
Net change in other loans held for sale					0	59,885,000
Increase in other assets					(87,405,000)	(12,397,000)
Increase (decrease) in other liabilities					48,269,000	(7,520,000)
Net cash provided by operating activities					44,045,000	29,882,000
Investing activities
Purchases of securities available for sale					(304,955,000)	(366,265,000)
Proceeds from sales of securities available for sale	0		199,875,000		8,773,000	212,485,000
Proceeds from call/maturities of securities available for sale					314,363,000	192,520,000
Net increase in loans					(1,383,382,000)	(93,761,000)
Purchases of premises and equipment					(6,824,000)	(23,968,000)
Proceeds from sales of premises and equipment					0	2,246,000
Net change in FHLB stock					10,607,000	6,389,000
Proceeds from sales of other assets					6,020,000	17,250,000
Other, net					0	917,000
Net cash used in investing activities					(1,355,398,000)	(52,187,000)
Financing activities
Net increase in noninterest-bearing deposits					1,206,472,000	288,350,000
Net increase (decrease) in interest-bearing deposits					514,646,000	(129,873,000)
Net decrease in short-term borrowings					(446,467,000)	(182,104,000)
Proceeds from the issuance of long-term debt, net of issuance costs					98,299,000	0
Repayment of long-term debt					(246,000)	(33,631,000)
Cash paid for dividends					(37,607,000)	(38,242,000)
Repurchase of shares in connection with stock repurchase program					(24,569,000)	(41,645,000)
Net cash provided by (used in) financing activities					1,310,528,000	(137,145,000)
Net decrease in cash and cash equivalents					(825,000)	(159,450,000)
Cash and cash equivalents at beginning of period		$ 414,930,000		$ 569,111,000	414,930,000	569,111,000	569,111,000
Cash and cash equivalents at end of period	$ 414,105,000		$ 409,661,000		414,105,000	409,661,000	$ 414,930,000
Supplemental disclosures
Cash paid for interest					61,351,000	75,720,000
Cash paid for income taxes					26,148,000	25,892,000
Noncash transactions:
Transfers of loans to other real estate owned					7,887,000	3,613,000
Financed sales of other real estate owned					151,000	254,000
Recognition of operating right-of-use assets					4,151,000	89,770,000
Recognition of operating lease liabilities					4,151,000	93,289,000
Mortgage Receivable
Noncash transactions:
Transfer of loans held-for-sale to portfolio loans					0	189,000
Loans Receivable
Noncash transactions:
Transfer of loans held-for-sale to portfolio loans					$ 0	$ 134,335,000