Quarterly Operating Results (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 474,316	$ 475,646	$ 466,305	$ 465,080	$ 433,664	$ 462,274	$ 446,518	$ 424,173	$ 1,881,347	$ 1,766,629	$ 1,660,509
Operating Expenses	367,982	351,227	358,303	354,113	338,763	345,302	337,441	321,624	1,431,625	1,343,130	1,257,950
Income before income taxes	106,334	124,419	108,002	110,967	94,901	116,972	109,077	102,549	449,722	423,499	402,559
Net Income (Loss) Attributable to Parent	$ 187,505	$ 75,913	$ 66,102	$ 70,110	$ 57,626	$ 71,545	$ 66,250	$ 62,070	$ 399,630	$ 257,491	$ 243,318
Earnings Per Share, Basic	$ 0.0135	$ 0.0054	$ 0.0047	$ 0.0050	$ 0.0041	$ 0.0051	$ 0.0047	$ 0.0045	$ 2.86	$ 1.84	$ 1.72
Diluted (in dollars per share)	$ 0.0132	$ 0.0053	$ 0.0046	$ 0.0049	$ 0.0041	$ 0.0050	$ 0.0047	$ 0.0044	$ 2.81	$ 1.82	$ 1.70