Debt and Lease Obligations (Senior Credit Facility) (Details)		3 Months Ended	12 Months Ended
	Oct. 14, 2015	Mar. 31, 2017	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)	Oct. 04, 2013 USD ($)
Line of Credit Facility [Line Items]
Repayments of Long-term Debt			$ 60,000,000	$ 60,000,000	$ 45,000,000
Term Loan
Line of Credit Facility [Line Items]
Debt Instrument, Face Amount							$ 400,000,000
Repayments of Long-term Debt			60,000,000	60,000,000	$ 45,000,000
Credit Facilities Scheduled Repayments Due in Next four Quarters			15,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity			$ 800,000,000				1,000,000,000
Debt Instrument, Maximum Permitted Leverage Ratio Debt to Total Consolidated EBITDA		3.50
Debt Instrument, Amended Maximum Permitted Leverage Ratio Debt to Total Consolidated EBITDA			4.00
Credit Agreement Maximum Borrowing Capacity in Additional Borrowings				400,000,000
Line of Credit Facility, Amount Outstanding			$ 0	0
Line of Credit Facility, Current Borrowing Capacity			$ 644,800,000	553,500,000
Debt Instrument, Amended Step-down Maximum Permitted Leverage Ratio Debt to Total Consolidated EBITDA			3.75
Interest Rate Margin, LIBOR Loans			0.00%
Interest rate margin, base rate loans			0.00%
Maximum Principal Amount of Priority Debt Allowed		15.00%
Amended Maximum Principal Amount of Priority Debt Allowed						7.50%
Maximum Amount of Securitzed Receivable							$ 200,000,000
Amended Maximum Amount of Securitzed Receivable						$ 100,000,000
Senior Credit Facility
Line of Credit Facility [Line Items]
Amended Debt Instrument Maturity Date	Oct. 14, 2020
Line of Credit Facility, Commitment Fee Percentage			0.20%
Letters of Credit Outstanding, Amount			$ 94,800,000	$ 102,600,000