Short and Long-Term Debt and Commitments Related to Letters of Credit Narrative (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 07, 2017 USD ($)
Debt instrument [Line Items]
Committed under credit facility	$ 7,097,000
Revolving credit facility, current borrowing capacity	250,000,000
Revolving credit facility, maximum borrowing capacity	325,000,000
Swing line commitment	$ 25,000,000
Number of principal payments	5
Anniversary when principal payments begin	6
Debt Instrument, Annual Principal Payment	$ 10,000,000
Unused commitments under credit facility	153,377,000
Revolving credit facility, noncurrent	$ 93,500,000	$ 100,000,000
Long-term debt, weighted average interest rate	4.28%	3.56%
Long-term debt, fiscal year maturity [Abstract]
2019	$ 22,514,000
2020	7,569,000
2021	5,093,000
2022	93,500,000
2023	0
thereafter year five	$ 50,000,000
Minimum [Member]
Debt instrument [Line Items]
Line of credit facility, commitment fee percentage	0.10%
Maximum [Member]
Debt instrument [Line Items]
Line of credit facility, commitment fee percentage	0.225%
Line of credit [Member]
Debt instrument [Line Items]
Revolving credit facility, noncurrent	$ 96,623,000
Letter of credit [Member]
Debt instrument [Line Items]
Letters of credit outstanding	$ 72,565,000	$ 63,242,000
London Interbank Offered Rate (LIBOR) [Member] | Line of credit [Member]
Debt instrument [Line Items]
Outstanding borrowings interest rate margin adjustment range	1.50%
London Interbank Offered Rate (LIBOR) [Member] | Line of credit [Member] | Minimum [Member]
Debt instrument [Line Items]
Outstanding borrowings interest rate margin adjustment range	0.875%
London Interbank Offered Rate (LIBOR) [Member] | Line of credit [Member] | Maximum [Member]
Debt instrument [Line Items]
Outstanding borrowings interest rate margin adjustment range	1.50%
Base rate [Member] | Line of credit [Member]
Debt instrument [Line Items]
Outstanding borrowings interest rate margin adjustment range	0.50%
Line of credit [Member] | Letter of credit [Member]
Debt instrument [Line Items]
Letters of credit outstanding	$ 2,004,000	2,004,000
5.09% Senior unsecured notes [Member]
Debt instrument [Line Items]
Senior unsecured notes	$ 12,000,000	18,000,000
Senior unsecured notes, interest rate	5.09%
Debt Instrument, Face Amount	$ 30,000,000
3.99% Senior unsecured notes [Member]
Debt instrument [Line Items]
Senior unsecured notes	$ 50,000,000	0
Senior unsecured notes, interest rate	3.99%
Debt Instrument, Face Amount	$ 50,000,000
Unsecured debt additional shelf note capacity	50,000,000
Unsecured debt maximum borrowing capacity allowed by credit facility	$ 75,000,000
Revolving credit facility [Member] | Minimum [Member]
Debt instrument [Line Items]
Interest coverage ratio	3
Revolving credit facility [Member] | Maximum [Member]
Debt instrument [Line Items]
Ratio of Indebtedness to Net Capital	3
Revolving credit facility [Member] | Letter of credit [Member]
Debt instrument [Line Items]
Letters of credit outstanding	$ 3,123,000	3,088,000
Accounts receivable securitization [Member]
Debt instrument [Line Items]
Senior unsecured notes, interest rate	0.90%
Long-term debt, fiscal year maturity [Abstract]
Unused commitment fee threshold percent	0.50
Debt instrument variable rate base rate calculation, default rate	2.00%
Accounts receivable securitization [Member] | Letter of credit [Member]
Debt instrument [Line Items]
Revolving credit facility, maximum borrowing capacity			$ 100,000,000
Letters of credit outstanding	$ 67,438,000	$ 58,150,000
Long-term debt, fiscal year maturity [Abstract]
Debt instrument, term (in years)	1 year
Accounts receivable securitization [Member] | Letter of credit [Member] | Minimum [Member]
Debt instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Accounts receivable securitization [Member] | Letter of credit [Member] | Maximum [Member]
Debt instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.45%
Accounts receivable securitization [Member] | Base rate, federal funds [Member]
Debt instrument [Line Items]
Outstanding borrowings interest rate margin adjustment range	0.50%
Line of credit [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Debt instrument [Line Items]
Outstanding borrowings interest rate margin adjustment range	0.75%
Line of credit [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Debt instrument [Line Items]
Outstanding borrowings interest rate margin adjustment range	1.50%
Letter of credit [Member]
Debt instrument [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 100,000,000