Loans (Analysis of Loan Portfolio by Major Types of Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	$ 8,933,321		$ 8,743,465
Financing Receivable, Allowance for Credit Loss	(122,074)	$ (85,600)	(83,968)	$ (83,274)	$ (83,369)
Loans and Leases Receivable, Net Amount	8,811,247		8,659,497
Loans held for sale	9,701		17,718
Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,969,974		3,945,853
Financing Receivable, Allowance for Credit Loss	(37,122)	(27,873)	(20,340)	(16,359)	(14,766)
Commercial Business [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	3,169,668		2,989,613
Financing Receivable, Allowance for Credit Loss	(45,570)	(31,054)	(30,292)	(34,545)	(34,658)
Agriculture [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	754,491		765,371
Financing Receivable, Allowance for Credit Loss	(11,085)	(6,510)	(15,835)	(10,690)	(9,589)
Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	308,186		361,533
Financing Receivable, Allowance for Credit Loss	(8,845)	(6,821)	(8,571)	(11,965)	(14,395)
One-to-Four Family Residential [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	690,506		637,325
Financing Receivable, Allowance for Credit Loss	(17,659)	(11,672)	(7,435)	(8,311)	(8,024)
Consumer Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Loans and Leases Receivable, Net of Deferred Income	40,496		43,770
Financing Receivable, Allowance for Credit Loss	$ (1,644)	$ (1,661)	$ (883)	$ (823)	$ (787)