Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation trust assets	$ 45,824	$ 42,161
Fair value of interest rate derivatives	138	(194)
Deferred compensation plan, asset	45,902	42,296
Deferred compensation liability	35,911	33,287
Deferred compensation plan, liability	36,345	33,357
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation trust assets	0	0
Deferred compensation plan, asset	0	0
Deferred compensation liability	0	0
Deferred compensation plan, liability	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation trust assets	45,824	42,161
Deferred compensation plan, asset	45,902	42,296
Deferred compensation liability	35,911	33,287
Deferred compensation plan, liability	36,345	33,357
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation trust assets	0	0
Deferred compensation plan, asset	0	0
Deferred compensation liability	0	0
Deferred compensation plan, liability	$ 0	$ 0
Interest Rate Swap No. 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative, term of contract	5 years	5 years
Fair value of interest rate derivatives	$ 78	$ 135
Interest Rate Swap No. 2 [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of interest rate derivatives	$ 0	$ 0
Interest Rate Swap No. 2 [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative, term of contract	5 years	5 years
Fair value of interest rate derivatives	$ 78	$ 135
Interest Rate Swap No. 2 [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of interest rate derivatives	$ 0	$ 0
Interest Rate Swap No. 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative, term of contract	10 years	10 years
Fair value of interest rate derivatives	$ (434)	$ (70)
Fair value of interest rate derivatives	434	70
Interest Rate Swap No. 1 [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of interest rate derivatives	$ 0	$ 0
Interest Rate Swap No. 1 [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative, term of contract	10 years	10 years
Fair value of interest rate derivatives	$ 434	$ 70
Interest Rate Swap No. 1 [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of interest rate derivatives	$ 0	$ 0