Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 106,521	$ 95,636	$ 86,795
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for loan losses	11,053	9,991	6,822
Net (gain) loss on disposal of property and equipment	137	621	(731)
Depreciation and amortization	18,432	19,480	18,313
Net premium amortization on investment securities	11,616	12,520	14,880
Net (gain) loss on investment securities transactions	(718)	(330)	(137)
Realized and unrealized net gains on mortgage banking activities	(25,174)	(26,806)	(21,748)
Net loss (gain) on sale of OREO	59	(925)	(2,991)
Write-down of OREO and other assets pending disposal	406	792	1,013
Net (gain) on sale of Health Savings Accounts	(3,068)	0	0
Mortgage servicing rights recovery	(93)	(35)	(97)
Deferred income tax expense (benefit)	20,917	3,369	12,449
Net increase in cash surrender value of company-owned life insurance policies	(5,397)	(4,477)	(3,432)
Stock-based compensation expense	3,868	4,376	3,959
Tax benefits from stock-based compensation	0	2,146	1,443
Excess tax benefits from stock-based compensation	0	(1,566)	(1,184)
Originations of mortgage loans held for sale	(945,200)	(1,113,292)	(1,148,098)
Proceeds from sales of mortgage loans held for sale	990,266	1,125,365	1,164,804
Changes in operating assets and liabilities:
(Increase) decrease in interest receivable	(574)	(1,053)	(101)
Decrease (increase) in other assets	(6,063)	(2,214)	(3,622)
Increase (decrease) in interest payable	178	445	(1,006)
Increase (decrease) in accounts payable and accrued expenses	(22,483)	(5,973)	(11,931)
Net cash provided by operating activities	154,683	118,070	115,400
Purchases of investment securities:
Held-to-maturity	(12,793)	(18,173)	(45,179)
Available-for-sale	(614,267)	(905,940)	(474,846)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	97,407	112,563	118,406
Available-for-sale	426,221	814,598	648,683
Purchase of company-owned life insurance	0	0	(30,000)
Extensions of credit to customers, net of repayments	(99,722)	(168,321)	(327,878)
Recoveries of loans charged-off	7,749	9,249	7,432
Proceeds from sales of OREO	5,913	5,325	15,644
Acquisition of intangible assets	(28,013)	0	0
Proceeds from the sale of Health Savings Accounts	6,138	0	0
Proceeds from sale of loan production office	0	932	0
Acquisition of bank and bank holding company, net of cash and cash equivalents, received	91,779	18,554	(1,636)
Capital expenditures, net of proceeds from sales	(10,977)	(11,879)	(5,965)
Net cash provided by (used in) investing activities	(130,565)	(143,092)	(95,339)
Cash flows from financing activities:
Net increase (decrease) in deposits	(110,216)	77,586	19,160
Net increase (decrease) in securities sold under repurchase agreements	105,405	8,871	6,385
Net increase (decrease) in other borrowed funds	64	4	(7)
Repayments of long-term debt	92	(65)	(16,538)
Advances on long-term debt	5,000	150	5,103
Write-off of debt issuance costs	0	0	7
Proceeds from issuance of common stock	2,431	4,683	3,369
Excess tax benefits from stock-based compensation	0	1,566	1,184
Purchase and retirement of common stock	(1,348)	(26,854)	(20,647)
Dividends paid to common stockholders	(48,583)	(39,353)	(36,290)
Net cash used in financing activities	(47,155)	26,588	(38,274)
Net change in cash and cash equivalents	(23,037)	1,566	(18,213)
Cash and cash equivalents at beginning of period	782,023	780,457	798,670
Cash and cash equivalents at end of period	758,986	782,023	780,457
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	28,759	54,419	27,335
Cash paid during the period for interest expense	27,757	17,200	18,933
Supplemental disclosures of noncash investing and financing activities:
Transfer of loans to loans held for sale	3,009	0	0
Transfer from long-term debt to other borrowed funds	20,000	0	0
Transfer of loans to other real estate owned	5,219	7,648	5,560
Capitalization of internally originated mortgage servicing rights	5,563	5,758	3,614
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	424,302	0	0
Investment securities held to maturity	57,307	0	0
Loans held for sale	10,253	0	0
Loans	2,079,336	0	0
Premises and equipment	46,732	0	0
Goodwill	231,886	0	0
Core deposit intangible	47,968	0	0
Mortgage servicing rights	3,491	0	0
Interest receivable	7,605	0	0
Company-owned life insurance	57,038	0	0
Deferred tax assets	28,640	0	0
Other real estate owned	1,192	0	0
Other assets	31,572	0	0
Total noncash assets acquired	3,027,322	0	0
Deposits	(2,668,976)	0	0
Accounts payable and accrued expenses	(64,156)	0	0
Total liabilities assumed	$ (2,733,132)	$ 0	$ 0