Short-Term Borrowing Short-Term Borrowing - Schedule of Short-Term Debt (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 370,000
Long-term Line of Credit		240,671	$ 201,311
Line of Credit Facility, Remaining Borrowing Capacity		129,329
Bank Overdrafts	[1]	10,298	8,560
Short-term borrowing		250,969	209,871
Committed Line of Credit Facility One [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 55,000
Line of Credit Facility, Expiration Date		Oct. 28, 2018
Long-term Line of Credit		$ 55,000	45,000
Line of Credit Facility, Remaining Borrowing Capacity		$ 0
Debt Instrument, Description of Variable Rate Basis		LIBOR rate, plus 1.00 percent
Short-term Revolving Line of Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 50,000
Line of Credit Facility, Expiration Date		Oct. 31, 2018
Long-term Line of Credit		$ 50,000	50,000
Debt Instrument, Description of Variable Rate Basis		Lender's base rate plus 80 basis points
Committed Line of Credit Facility Three [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 45,000
Line of Credit Facility, Expiration Date		Oct. 31, 2018
Long-term Line of Credit		$ 40,171	35,000
Line of Credit Facility, Remaining Borrowing Capacity		$ 4,829
Debt Instrument, Description of Variable Rate Basis		Lender's base rate, plus 0.85 percent
Committed Line of Credit Facility Five [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 40,000
Line of Credit Facility, Expiration Date		Oct. 31, 2018
Line of Credit Facility, Remaining Borrowing Capacity		$ 40,000
Debt Instrument, Description of Variable Rate Basis		Lender's base rate, plus 0.85 percent
Committed Line of Credit Facility Four [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	[2]	$ 150,000
Line of Credit Facility, Expiration Date		Oct. 08, 2020
Long-term Line of Credit	[2]	$ 75,000	50,000
Line of Credit Facility, Remaining Borrowing Capacity	[2]	$ 75,000
Debt Instrument, Description of Variable Rate Basis		LIBOR rate, plus 1.00 percent
Committed Line of Credit Facility Two [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 30,000
Line of Credit Facility, Expiration Date		Oct. 31, 2018
Long-term Line of Credit		$ 20,500	$ 21,311
Line of Credit Facility, Remaining Borrowing Capacity		$ 9,500
Debt Instrument, Description of Variable Rate Basis		LIBOR rate, plus 1.00 percent

[1] If presented, these book overdrafts would be funded through the bank revolving credit facilities.

[2] This committed revolving credit facility includes a restriction that our short-term borrowings, excluding any borrowings under the committed revolving credit facility, shall not exceed $200.0 million.