Debt Narratives (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Document Period End Date			Jun. 30, 2018
Debt, Weighted Average Interest Rate	4.88%		4.88%		4.73%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	66.00%		66.00%		75.00%
Interest Paid, Including Capitalized Interest, Operating and Investing Activities			$ 89,100	$ 79,900
Letters of Credit Outstanding, Amount	$ 33,300		$ 33,300
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage			0.25%
Line of Credit Facility, Remaining Borrowing Capacity	596,700		$ 596,700
Repayments of Long-term Debt			8,631	17,570
Gain (Loss) on Extinguishment of Debt	0	$ 0	(10,131)	0
Repayments of Debt			268,200	17,600
Repayments of Long-term Capital Lease Obligations			19,270	30,419
December 2022
Debt Instrument [Line Items]
Bank Credit Facility	370,000		370,000
Term Loan	666,563		666,563		$ 675,000
Letters of Credit Outstanding, Amount					0
Line of Credit Facility, Increase (Decrease), Net			185,000
Repayments of Long-term Debt			9,600
Repayments of Debt			8,400
Term Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Increase (Decrease), Net			10,000	$ 110,000
Repayments of Long-term Debt			17,500
Repayments of Debt			200
October 2018
Debt Instrument [Line Items]
Line of Credit Facility, Increase (Decrease), Net			175,000
Repayments of Long-term Debt			250,000
Unsecured Debt | May 2024
Debt Instrument [Line Items]
Senior Notes	850,000		850,000		850,000
Unsecured Debt | October 2018
Debt Instrument [Line Items]
Senior Notes	$ 0		$ 0		$ 250,000