Quarterly Operating Results (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 508,748	$ 530,850	$ 473,187	$ 501,461	$ 474,316	$ 475,646	$ 466,305	$ 465,080	$ 2,014,246	$ 1,881,347	$ 1,766,629
Operating Expenses	408,137	388,350	372,277	383,020	367,982	351,227	358,303	354,113	1,551,784	1,431,625	1,343,130
Income before income taxes	100,611	142,500	100,910	118,441	106,334	124,419	108,002	110,967	462,462	449,722	423,499
Net Income (Loss) Attributable to Parent	$ 73,452	$ 106,053	$ 73,922	$ 90,828	$ 187,505	$ 75,913	$ 66,102	$ 70,110	$ 344,255	$ 399,630	$ 257,491
Earnings Per Share, Basic	$ 0.0026	$ 0.0038	$ 0.0027	$ 0.0033	$ 0.0068	$ 0.0027	$ 0.0024	$ 0.0025	$ 1.24	$ 1.43	$ 0.92
Diluted (in dollars per share)	$ 0.0026	$ 0.0038	$ 0.0026	$ 0.0032	$ 0.0066	$ 0.0027	$ 0.0023	$ 0.0025	$ 1.22	$ 1.40	$ 0.91