Unaudited Quarterly 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 [Line Items]
Revenues	$ 1,582,447	$ 1,500,673	$ 1,470,116	$ 1,318,991	$ 1,226,827	$ 1,220,122	$ 1,212,629	$ 1,200,849	$ 5,872,227	$ 4,860,427	$ 4,103,728
Net income	$ 173,519	$ 388,317	$ 387,744	$ 248,931	$ 253,115	$ 264,804	$ 196,218	$ 181,385	$ 1,198,511	$ 895,522	$ 636,056
Net income per share - basic (in USD per shares)	$ 1.62	$ 3.64	$ 3.66	$ 2.36	$ 2.41	$ 2.53	$ 1.88	$ 1.74	$ 11.27	$ 8.55	$ 6.17
Net income per share - diluted (in USD per shares)	$ 1.50	$ 3.32	$ 3.34	$ 2.16	$ 2.19	$ 2.27	$ 1.69	$ 1.59	$ 10.34	$ 7.70	$ 5.52
Income Tax Expense (Benefit)									$ 880,000	$ 434,293	$ 589,041
Excess Tax Benefit from Share-based Compensation, Financing Activities									0	0	$ 405,317
Accounting Standards Update 2016-09 [Member] | New Accounting Pronouncement, Early Adoption, Effect [Member]
Quarterly Financial Information Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Financing Activities									191,000	$ 144,800
Accounting Standards Update 2016-09 [Member] | Additional Paid-in Capital | New Accounting Pronouncement, Early Adoption, Effect [Member]
Quarterly Financial Information Disclosure [Line Items]
Excess Tax Benefit from Share-based Compensation, Financing Activities								$ 15,600
Tarrytown lease transaction [Member]
Quarterly Financial Information Disclosure [Line Items]
Out of period adjustment			$ 30,100
2017 Tax Cut and Jobs Act [Member]
Quarterly Financial Information Disclosure [Line Items]
Remeasurement of Deferred Tax Assets due to US Tax Reform									$ 326,200