Loans (Analysis of Loan Portfolio by Major Types of Loans) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator - interest written off on nonaccrual loans [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 9,693,116		$ 9,427,660
Financing Receivable, Allowance for Credit Loss	(142,988)	$ (148,294)	(149,140)	$ (151,546)	$ (122,074)	$ (83,968)
Loans and Leases Receivable, Net Amount	9,550,128		9,278,520
Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator - interest written off on nonaccrual loans [Line Items]
Loans and Leases Receivable, Net of Deferred Income	4,101,071		4,062,313
Financing Receivable, Allowance for Credit Loss	(53,250)	(57,050)	(68,934)	(50,233)	(37,122)	(20,340)
Commercial Business [Member]
Financing Receivable, Credit Quality Indicator - interest written off on nonaccrual loans [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,738,288		3,597,968
Financing Receivable, Allowance for Credit Loss	(57,554)	(58,405)	(45,250)	(53,186)	(45,570)	(30,292)
Agriculture [Member]
Financing Receivable, Credit Quality Indicator - interest written off on nonaccrual loans [Line Items]
Loans and Leases Receivable, Net of Deferred Income	797,580		779,627
Financing Receivable, Allowance for Credit Loss	(7,920)	(9,487)	(9,052)	(14,868)	(11,085)	(15,835)
Construction [Member]
Financing Receivable, Credit Quality Indicator - interest written off on nonaccrual loans [Line Items]
Loans and Leases Receivable, Net of Deferred Income	300,303		268,663
Financing Receivable, Allowance for Credit Loss	(6,559)	(6,551)	(7,636)	(7,953)	(8,845)	(8,571)
One-to-Four Family Residential [Member]
Financing Receivable, Credit Quality Indicator - interest written off on nonaccrual loans [Line Items]
Loans and Leases Receivable, Net of Deferred Income	724,151		683,570
Financing Receivable, Allowance for Credit Loss	(16,519)	(15,638)	(16,875)	(23,711)	(17,659)	(7,435)
Consumer Loan [Member]
Financing Receivable, Credit Quality Indicator - interest written off on nonaccrual loans [Line Items]
Loans and Leases Receivable, Net of Deferred Income	31,723		35,519
Financing Receivable, Allowance for Credit Loss	$ (1,186)	$ (1,163)	$ (1,393)	$ (1,595)	$ (1,644)	$ (883)