Retirement and Compensation Plans Share based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options, Outstanding [Roll Forward]
Options, Outstanding, Number		134,848
Options, Exercises in Period		(89,453)
Options, Forfeitures in Period		0
Options, Outstanding, Number		45,395	134,848
Options, Vested and Expected to Vest, Outstanding, Number		45,395
Options, Exercisable, Number		45,395
Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Options, Outstanding, Weighted Average Exercise Price		$ 28.07
Options, Exercises in Period, Weighted Average Exercise Price		28.44
Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price		0.00
Options, Outstanding, Weighted Average Exercise Price		27.35	$ 28.07
Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price		27.35
Options, Exercisable, Weighted Average Exercise Price		$ 27.35
Options, Outstanding, Intrinsic Value	[1]	$ 2,306
Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value	[1]	2,306
Options, Exercisable, Intrinsic Value	[1]	2,306
Options, Exercises in Period, Total Intrinsic Value		3,200	$ 4,500	$ 6,500
Cash Received from Exercise of Stock Options		2,500	3,700	10,500
Tax Benefit Realized from Exercise of Stock Options		$ 1,000	1,400	2,200
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term		2 years 4 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term		2 years 4 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term		2 years 4 days
Class A common stock compensation plans [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated Share-based Compensation Expense				200
Tax Benefit from Compensation Expense				100
Cash-based restricted stock unit plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated Share-based Compensation Expense		$ 2,300	1,700	1,000
Tax Benefit from Compensation Expense		$ 1,200	$ 900	$ 600
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		1 year 10 months 3 days
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		145,415
Stock units, Grants in Period		25,075
Stock units, Vested in Period		(40,155)
Stock units, Forfeited in Period		(8,818)
Number of shares or units end of period		121,517	145,415
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Nonvested, Weighted Average Grant Date Fair Value		$ 46.10	$ 42.36
Stock units, Grants in Period, Weighted Average Grant Date Fair Value		60.34	$ 52.19	$ 38.63
Stock units, Vested in Period, Weighted Average Grant Date Fair Value		40.58
Stock units, Forfeitures, Weighted Average Grant Date Fair Value		$ 50.00
Nonvested Awards, Total Compensation Cost Not yet Recognized		$ 4,100
share based compensation restricted stock units cash paid expense		$ 2,700	$ 1,800	$ 1,800
Directors stock grants-cash based units [Member]
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		22,874
Number of shares or units end of period		24,923	22,874
Directors nonrestricted Class A stock grants [Member]
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		56,868
Number of shares or units end of period		57,876	56,868
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Number of Shares Available for Grant		110,864
Executive Salary and Bonus Deferred Compensation Plan-cash based units [Member]
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		16,679
Number of shares or units end of period		16,323	16,679
Executive Salary and Bonus Deferred Compensation Plan [Member]
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		66,621
Number of shares or units end of period		63,909	66,621
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Number of Shares Available for Grant		99,933
Executive excess 401(k) plan-cash based units [Member]
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		84
Number of shares or units end of period		89	84
Executive excess 401(k) plan [Member]
Nonvested & restricted Stock Units, Number of Shares [Roll Forward]
Number of shares or units beginning		2,991
Number of shares or units end of period		3,168	2,991

[1] Represents the difference between the share price and exercise price for each option, excluding options where the exercise price is above the share price, at December 31, 2016.