Consolidated Statements of Cash Flows (Unaudited) - USD ($)	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Operating activities
Net income	$ 37,446,000	$ 45,110,000	$ 31,964,000	$ 33,826,000	$ 129,181,000	$ 102,500,000
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	1,700,000		2,250,000		4,100,000	5,810,000
Depreciation, amortization and accretion					5,826,000	3,689,000
Deferred income tax expense					13,911,000	7,335,000
Funding of mortgage loans held for sale					(1,680,729,000)	(1,318,484,000)
Proceeds from sales of mortgage loans held for sale					1,543,544,000	1,253,680,000
Gains on sales of mortgage loans held for sale					(35,416,000)	(30,805,000)
Valuation adjustment to mortgage servicing rights					3,132,000	0	$ 0
(Gains) losses on sales of securities	(343,000)		16,000		(348,000)	16,000
Penalty on prepayment of debt	54,000		0		54,000	0
Gains on sales of premises and equipment					(1,062,000)	(188,000)
Stock-based compensation expense					7,721,000	5,556,000
Net change in other loans held for sale					59,885,000	0
Increase in other assets					(12,397,000)	(57,000)
Decrease in other liabilities					(7,520,000)	(27,084,000)
Net cash provided by operating activities					29,882,000	1,968,000
Investing activities
Purchases of securities available for sale					(366,265,000)	(576,579,000)
Proceeds from sales of securities available for sale	199,875,000		2,387,000		212,485,000	2,387,000
Proceeds from call/maturities of securities available for sale					192,520,000	113,511,000
Net increase in loans					(93,761,000)	(156,082,000)
Purchases of premises and equipment					(23,968,000)	(15,599,000)
Proceeds from sales of premises and equipment					2,246,000	912,000
Net change in FHLB stock					6,389,000	0
Proceeds from sales of other assets					17,250,000	5,286,000
Net cash received in acquisition of businesses					0	153,502,000
Other, net					917,000	0
Net cash used in investing activities					(52,187,000)	(472,662,000)
Financing activities
Net increase in noninterest-bearing deposits					288,350,000	90,240,000
Net (decrease) increase in interest-bearing deposits					(129,873,000)	448,675,000
Net (decrease) increase in short-term borrowings					(182,104,000)	51,606,000
Repayment of long-term debt					(33,631,000)	(643,000)
Cash paid for dividends					(38,242,000)	(31,242,000)
Repurchase of shares in connection with stock repurchase program					(41,645,000)	0
Net stock-based compensation transactions					0	201,000
Net cash (used in) provided by financing activities					(137,145,000)	558,837,000
Net (decrease) increase in cash and cash equivalents					(159,450,000)	88,143,000
Cash and cash equivalents at beginning of period		$ 569,111,000		$ 281,453,000	569,111,000	281,453,000	281,453,000
Cash and cash equivalents at end of period	$ 409,661,000		$ 369,596,000		409,661,000	369,596,000	$ 569,111,000
Supplemental disclosures
Cash paid for interest					75,720,000	43,317,000
Cash paid for income taxes					25,892,000	21,305,000
Noncash transactions:
Transfers of loans to other real estate owned					3,613,000	2,657,000
Financed sales of other real estate owned					254,000	495,000
Common stock issued in acquisition of businesses					0	434,519,000
Recognition of operating right-of-use assets					89,770,000
Recognition of operating lease liabilities					93,289,000
Mortgage receivable
Noncash transactions:
Transfers of mortgage loans held for sale to loans held for investment					189,000	1,510,000
Loans receivable
Noncash transactions:
Transfers of mortgage loans held for sale to loans held for investment					$ 134,335,000	$ 0