Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 44,138,000	$ 49,456,000	$ 42,024,000
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	9,425,000	9,553,000	9,766,000
Accretion of acquired loans	(449,000)	(686,000)	(1,373,000)
Provision for loan losses	14,050,000	1,750,000	1,100,000
Deferred income tax expense (benefit)	(4,141,000)	26,000	(372,000)
Stock-based compensation expense	2,325,000	2,931,000	2,410,000
Stock grants to directors for retainer fee	394,000	375,000	441,000
Proceeds from sales of mortgage loans held for sale	677,634,000	261,021,000	101,146,000
Origination of mortgage loans held for sale	(665,992,000)	(256,767,000)	(96,179,000)
Net gain on sales of mortgage loans held for sale	(29,541,000)	(8,108,000)	(3,536,000)
Net gain from sales and valuation write-downs of foreclosed assets	(325,000)	(254,000)	(169,000)
Net gain from sale and write-downs on former bank premises	(27,000)	(436,000)	(78,000)
Net loss from sales and disposals of premises and equipment	1,514,000	294,000	134,000
Earnings on bank owned life insurance	(1,214,000)	(3,886,000)	(1,287,000)
Net change in:
Accrued interest receivable	(917,000)	(48,000)	(1,126,000)
Other assets	(10,892,000)	(7,636,000)	(2,193,000)
Accrued interest and other liabilities	1,895,000	(2,818,000)	11,029,000
Net cash from operating activities	37,877,000	44,767,000	61,737,000
Cash flows from investing activities
Purchases of securities available for sale	(369,682,000)	(62,084,000)	(48,664,000)
Proceeds from maturities, calls and repayments of securities available for sale	321,164,000	79,478,000	40,308,000
Purchases of Federal Home Loan Bank stock	0	(1,980,000)	(4,986,000)
Loan originations and payments, net	(341,461,000)	(99,620,000)	(193,556,000)
Purchases of bank owned life insurance	(700,000)	(4,500,000)	0
Proceeds from bank owned life insurance cash value release and death benefits	0	7,708,000	0
Purchases of premises and equipment, net	(8,989,000)	(13,484,000)	(6,318,000)
Proceeds from sales of former bank premises	162,000	854,000	1,964,000
Proceeds from sales of foreclosed assets	700,000	790,000	772,000
Net cash for investing activities	(398,806,000)	(92,838,000)	(210,480,000)
Cash flows from financing activities
Net (decrease) increase in time deposits	(99,294,000)	170,921,000	(30,091,000)
Net (decrease) increase in all other deposits	820,463,000	55,755,000	(28,566,000)
Net (decrease) increase in securities sold under agreements to repurchase	15,690,000	(844,000)	(15,229,000)
Proceeds from Federal Home Loan Bank advances	60,000,000	44,000,000	160,000,000
Maturities of Federal Home Loan Bank advances	(20,000,000)	(40,000,000)	(30,000,000)
Proceeds from stock option exercises, net of cashless exercises	3,000	128,000	108,000
Employee stock purchase plan	49,000	50,000	52,000
Dividend reinvestment plan	814,000	729,000	1,165,000
Repurchases of common stock	(6,591,000)	(7,183,000)	(5,943,000)
Payment of cash dividends to common shareholders	(17,930,000)	(17,108,000)	(27,500,000)
Net cash from financing activities	753,204,000	206,448,000	23,996,000
Net change in cash and cash equivalents	392,275,000	158,377,000	(124,747,000)
Cash and cash equivalents at beginning of period	233,731,000	75,354,000	200,101,000
Cash and cash equivalents at end of period	626,006,000	233,731,000	75,354,000
Supplemental disclosures of cash flows information
Interest	27,703,000	32,103,000	21,569,000
Federal income taxes	10,950,000	11,975,000	10,075,000
Noncash financing and investing activities:
Transfers from loans to foreclosed assets	146,000	337,000	744,000
Transfers from bank premises to other real estate owned	$ 613,000	$ 258,000	$ 296,000