Business Segments and Geographic Information (Geographic Information) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Geographical Information
Revenues	$ 599,430	$ 583,530	$ 583,509	$ 564,917	$ 568,977	$ 566,759	$ 577,070	$ 550,614	$ 2,331,386	$ 2,263,420	$ 2,179,490
Property and equipment, net	2,158,610				2,078,123				2,158,610	2,078,123	1,897,241
U.S. operations
Geographical Information
Revenues									2,073,286	1,952,210	1,770,273
Property and equipment, net	2,145,197				2,061,711				2,145,197	2,061,711	1,887,043
Australia operations
Geographical Information
Revenues									231,164	285,702	388,361
Property and equipment, net	13,006				16,281				13,006	16,281	10,053
South African operations
Geographical Information
Revenues									19,806	18,251	13,658
Property and equipment, net	88				131				88	131	145
United Kingdom operations
Geographical Information
Revenues									7,130	7,257	7,198
Property and equipment, net	$ 319				$ 0				$ 319	$ 0	$ 0