Debt Narratives (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Document Period End Date					Sep. 30, 2018
Debt, Weighted Average Interest Rate	4.93%				4.93%		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					$ 113,100	$ 101,700
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	571,700				$ 571,700
Line of Credit Facility, Increase (Decrease), Net	25,000				395,000
Repayments of Long-term Debt					25,601	26,376
Gain (Loss) on Extinguishment of Debt	0			$ 0	(10,131)	0
Repayments of Debt					285,200	26,400
Repayments of Long-term Capital Lease Obligations					29,771	40,509
December 2022
Debt Instrument [Line Items]
Bank Credit Facility	395,000				395,000
Term Loan	649,688				649,688		$ 675,000
Letters of Credit Outstanding, Amount							0
Line of Credit Facility, Increase (Decrease), Net		$ 185,000	$ 10,000
Repayments of Long-term Debt					9,600
Repayments of Debt					25,300
Term Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Increase (Decrease), Net						$ 120,000
Repayments of Long-term Debt					26,300
Repayments of Debt					300
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