Fair Value (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans Receivable, Fair Value Disclosure	$ 6,478,525,000		$ 6,441,458,000
expense related to other repossessed assets	0	$ 0
Alternative Investment	20,521,000		20,411,000
Loans Held-for-sale, Fair Value Disclosure	23,901,000		12,278,000
Impaired Financing Receivable, Carrying Value	$ 80,320,000		72,261,000
Discount percentage applied to real estate appraised values	15.00%
Discount percentage applied to lot development appraised values	6.00%
Partial Charge-Offs On Impaired Loans	$ 733,000		719,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	5,531,000		5,230,000
Impaired Financing Receivable, Related Allowance	5,531,000		5,230,000
Impaired Loan Fair Value	2,154,000		2,090,000
Partial Charge-Offs On Impaired Loans carried at Fair Value	313,000		313,000
Book Value Of Impaired Loans Carried At Fair Value	2,275,000		2,167,000
Impaired Financing Receivable, carried at fair value, related expense	57,000	33,000
MSR recorded at lower of cost or fair value	8,768,000	10,082,000	10,070,000	$ 10,178,000
Mortgage Servicing Rights Fair Value			5,800,000
Valuation allowance of MSR	2,351,000	289,000	825,000	$ 232,000
MSRs recorded at cost	6,000		4,300,000
Changes due to fair value adjustments	(1,526,000)	(57,000)
Other Real Estate, Foreclosed Assets, and Repossessed Assets	$ 3,600,000		$ 4,029,000
Percent of OREO held at fair value	83.00%		75.00%
Other Real Estate, Foreclosed Assets, and Repossessed Assets, Fair Value	$ 3,000,000.0		$ 3,000,000.0
OREO valuation adjustments	2,000	27,000
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	7,800,000		8,500,000
Alternative Investment, income recognized	(204,000)	$ 1,600,000
Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Alternative Investment	12,700,000		11,900,000
Commercial Receivables, excluding Commercial, Financial, and Agricultural [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impaired Financing Receivable, Related Allowance	121,000		77,000
Financing Receivable, not collateral dependent [Domain]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impaired Financing Receivable, Related Allowance	5,410,000		5,153,000
Fair Value, Nonrecurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans Receivable, Fair Value Disclosure	$ 2,154,000		$ 2,090,000