Quarterly Operating Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 433,664	$ 462,274	$ 446,518	$ 424,173	$ 404,597	$ 432,167	$ 419,447	$ 404,298	$ 1,766,629	$ 1,660,509	$ 1,575,796
Operating Expenses	338,763	345,302	337,441	321,624	309,560	319,337	318,533	310,520	1,343,130	1,257,950	1,236,047
Income before income taxes	94,901	116,972	109,077	102,549	95,037	112,830	100,914	93,778	423,499	402,559	339,749
Net Income (Loss) Attributable to Parent	$ 57,626	$ 71,545	$ 66,250	$ 62,070	$ 57,935	$ 67,427	$ 61,005	$ 56,951	$ 257,491	$ 243,318	$ 206,896
Earnings Per Share, Basic	$ 0.0041	$ 0.0051	$ 0.0047	$ 0.0045	$ 0.0041	$ 0.0048	$ 0.0043	$ 0.0040	$ 1.84	$ 1.72	$ 1.43
Diluted (in dollars per share)	$ 0.0041	$ 0.0050	$ 0.0047	$ 0.0044	$ 0.0041	$ 0.0047	$ 0.0043	$ 0.0039	$ 1.82	$ 1.70	$ 1.41