Business Segments and Geographic Information (Geographic Information) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Geographical Information
Revenues	$ 568,977	$ 566,759	$ 577,070	$ 550,614	$ 566,579	$ 554,376	$ 548,350	$ 510,185	$ 2,263,420	$ 2,179,490	$ 1,843,307
Property and equipment, net	2,078,123				1,897,241				2,078,123	1,897,241	1,916,386
U.S. operations
Geographical Information
Revenues									1,952,210	1,770,273	1,581,811
Property and equipment, net	2,061,711				1,887,043				2,061,711	1,887,043	1,910,378
Australia operations
Geographical Information
Revenues									285,702	388,361	237,731
Property and equipment, net	16,281				10,053				16,281	10,053	5,871
South African operations
Geographical Information
Revenues									18,251	13,658	14,964
Property and equipment, net	131				145				131	145	90
United Kingdom operations
Geographical Information
Revenues									7,257	7,198	8,801
Property and equipment, net	$ 0				$ 0				$ 0	$ 0	$ 47