Retirement and Compensation Plans Share based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Options, Outstanding [Roll Forward]
Options, Outstanding, Number		11,176
Options, Exercises in Period		(4,480)
Options, Forfeitures in Period		0
Options, Outstanding, Number		6,696	11,176
Options, Vested and Expected to Vest, Outstanding, Number		6,696
Options, Exercisable, Number		6,696
Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Options, Outstanding, Weighted Average Exercise Price		$ 25.11
Options, Exercises in Period, Weighted Average Exercise Price		25.70
Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price		0
Options, Outstanding, Weighted Average Exercise Price		24.71	$ 25.11
Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price		24.71
Options, Exercisable, Weighted Average Exercise Price		$ 24.71
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term		7 months 10 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term		7 months 10 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term		7 months 10 days
Options, Outstanding, Intrinsic Value	[1]	$ 229
Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value	[1]	229
Options, Exercisable, Intrinsic Value	[1]	229
Options, Exercises in Period, Total Intrinsic Value		100	$ 700	$ 800
Cash Received from Exercise of Stock Options		100	300	700
Share-based Payment Arrangement, Exercise of Option, Tax Benefit		100	100	200
Cash-based restricted stock unit plan
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based Payment Arrangement, Expense		1,200	1,700	1,900
Tax Benefit from Compensation Expense		$ 400	$ 600	$ 1,000
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		77,191
Stock units, Grants in Period		23,982
Stock units, Vested in Period		(30,806)
Stock units, Forfeited in Period		(6,350)
Number of shares or units end of period		64,017	77,191
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Nonvested, Weighted Average Grant Date Fair Value		$ 67.30	$ 60.76
Stock units, Grants in Period, Weighted Average Grant Date Fair Value		70.88	$ 71.20	$ 69.10
Stock units, Vested in Period, Weighted Average Grant Date Fair Value		53.85
Stock units, Forfeitures, Weighted Average Grant Date Fair Value		$ 66.56
Nonvested Awards, Total Compensation Cost Not yet Recognized		$ 1,400
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition		2 years 11 days
share based compensation restricted stock units cash paid expense		$ 2,500	$ 3,300	$ 3,300
Directors stock grants-cash based units
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		25,016
Number of shares or units end of period		26,042	25,016
Directors nonrestricted Class A stock grants
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		50,194
Number of shares or units end of period		47,092	50,194
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Number of Shares Available for Grant		103,003
Executive Salary and Bonus Deferred Compensation Plan-cash based units
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		10,743
Number of shares or units end of period		8,478	10,743
Executive Salary and Bonus Deferred Compensation Plan
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		48,312
Number of shares or units end of period		48,740	48,312
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Number of Shares Available for Grant		92,641
Executive excess 401(k) plan
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		3,327
Number of shares or units end of period		3,514	3,327

[1] Represents the difference between the share price and exercise price for each option, excluding options for which the exercise price is above the share price, at December 31, 2019.