Debt Narratives (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Document Period End Date	Mar. 31, 2018
Debt, Weighted Average Interest Rate	4.90%		4.73%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	70.00%		75.00%
Interest Paid	$ 24,900	$ 20,000
Letters of Credit Outstanding, Amount	$ 33,300
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.25%
Line of Credit Facility, Remaining Borrowing Capacity	$ 781,700
Repayments of Long-term Debt	8,535	8,787
Gain (Loss) on Extinguishment of Debt	(10,131)	0
Repayments of Debt	268,100	8,800
Repayments of Long-term Capital Lease Obligations	7,646	21,055
December 2022
Debt Instrument [Line Items]
Bank Credit Facility	185,000
Term Loan	666,563		$ 675,000
Letters of Credit Outstanding, Amount			0
Line of Credit Facility, Increase (Decrease), Net		10,000
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	$ 25,000
Repayments of Debt	100
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
Unsecured Debt | October 2018
Debt Instrument [Line Items]
Senior Notes	$ 0		$ 250,000