Segment and Geographic Area Information Geographic Area Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 2,208,044	$ 2,001,527	$ 2,000,401
Percent of total operating revenues	100.00%	100.00%	100.00%
Property and equipment, net	$ 4,656,921	$ 4,503,319
Percent of total property and equipment	100.00%	100.00%
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 1,176,674	$ 1,142,141	$ 1,143,056
Percent of total operating revenues	53.30%	57.10%	57.10%
Property and equipment, net	$ 3,404,204	$ 3,353,296
Percent of total property and equipment	73.10%	74.40%
Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 307,510	$ 222,586	$ 242,952
Percent of total operating revenues	13.90%	11.10%	12.10%
Property and equipment, net	$ 664,367	$ 634,148
Percent of total property and equipment	14.30%	14.10%
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 97,611	$ 94,623	$ 98,759
Percent of total operating revenues	4.40%	4.70%	5.00%
Property and equipment, net	$ 253,597	$ 237,328
Percent of total property and equipment	5.40%	5.30%
U.K. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 475,239	$ 378,551	$ 340,747
Percent of total operating revenues	21.50%	18.90%	17.00%
Property and equipment, net	$ 317,495	$ 264,954
Percent of total property and equipment	6.80%	5.90%
Netherlands [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 78,819	$ 101,837	$ 119,421
Percent of total operating revenues	3.60%	5.10%	6.00%
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 72,191	$ 61,789	$ 55,466
Percent of total operating revenues	3.30%	3.10%	2.80%
Property and equipment, net	$ 17,258	$ 13,593
Percent of total property and equipment	0.40%	0.30%
Non-US [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
OPERATING REVENUES	$ 1,031,370	$ 859,386	$ 857,345
Percent of total operating revenues	46.70%	42.90%	42.90%
Property and equipment, net	$ 1,252,717	$ 1,150,023
Percent of total property and equipment	26.90%	25.60%