Debt Covenant Ratios (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Jun. 30, 2018
Credit Agreement
Debt Instrument [Line Items]
Net total lease adjusted leverage ratio	5.0	5.6
Net secured debt lease adjusted leverage ratio	1.6	2.5
Bond leverage ratio, per indentures	5.8	5.7
Fixed charge coverage ratio	2.1	2.3
Credit Agreement | Maximum
Debt Instrument [Line Items]
Net total lease adjusted leverage ratio		6.5
Net secured debt lease adjusted leverage ratio		4.0
Bond leverage ratio, per indentures		6.5
Credit Agreement | Minimum
Debt Instrument [Line Items]
Fixed charge coverage ratio		1.5
Senior Notes 4.875 Percent due 2027, GBP Notes due 2025 and 5.25 Percent Notes [Member]
Debt Instrument [Line Items]
Debt Instrument Covenant, Indenture, Leverage Ratio Maximum		7.0
Revolving Credit Facility
Debt Instrument [Line Items]
Repayments of Debt	$ 807,000