Basis of Presentation and Significant Accounting Policies - Effects of Adopting Topic 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Balance Sheets
Other current assets	$ 258,685		$ 258,685		$ 241,029	$ 232,027
Total current assets	1,806,987		1,806,987			2,249,128
Other assets	562,332		562,332		421,328	241,750
Total assets	20,288,071		20,288,071			18,691,457
Accounts payable and accrued expenses	739,117		739,117			719,257
Other current liabilities	123,824		123,824		143,699	159,914
Total current liabilities	1,461,319		1,461,319			1,242,734
Other liabilities	634,060		634,060		598,659	661,710
Total liabilities	13,127,898		13,127,898			11,841,667
Accumulated other comprehensive loss	(922,148)		(922,148)		(787,119)	(785,189)
Retained earnings	779,926		779,926		522,465	252,689
Total stockholders' equity	7,160,173		7,160,173			6,849,790
Total liabilities and stockholders' equity	20,288,071		20,288,071			$ 18,691,457
Statements of Operations
Revenues	1,283,751	$ 1,152,261	3,761,571	$ 3,168,207
Sales and marketing	157,920	157,619	471,898	428,112
Total costs and operating expenses	1,017,998	927,398	3,054,905	2,591,236
Income from operations	265,753	224,863	706,666	576,971
Income before income taxes	143,335		296,962
Income tax expense	(18,510)	(2,194)	(41,625)	(24,912)
Net income	$ 124,825	$ 79,900	$ 255,337	$ 167,767
Basic EPS (in dollars per share)	$ 1.56	$ 1.02	$ 3.21	$ 2.20
Diluted EPS (in dollars per share)	$ 1.55	$ 1.02	$ 3.19	$ 2.18
Statements of Cash Flow
Income taxes, net			$ (32,876)	$ (53,608)
Other assets			(22,409)	29,046
Other liabilities			14,879	6,293
Net cash provided by operating activities			1,256,797	$ 944,354
Calculated under revenue guidance in effect before Topic 606 [Member]
Balance Sheets
Other current assets	$ 249,327		249,327
Total current assets	1,797,629		1,797,629
Other assets	372,056		372,056
Total assets	20,088,437		20,088,437
Accounts payable and accrued expenses	736,007		736,007
Other current liabilities	142,460		142,460
Total current liabilities	1,476,845		1,476,845
Other liabilities	702,056		702,056
Total liabilities	13,211,420		13,211,420
Accumulated other comprehensive loss	(915,092)		(915,092)
Retained earnings	489,714		489,714
Total stockholders' equity	6,877,017		6,877,017
Total liabilities and stockholders' equity	20,088,437		20,088,437
Statements of Operations
Revenues	1,280,565		3,752,028
Sales and marketing	163,122		484,778
Total costs and operating expenses	1,023,200		3,067,785
Income from operations	257,365		684,243
Income before income taxes	134,947		274,539
Income tax expense	(19,043)		(39,638)
Net income	$ 115,904		$ 234,901
Basic EPS (in dollars per share)	$ 1.45		$ 2.95
Diluted EPS (in dollars per share)	$ 1.44		$ 2.94
Statements of Cash Flow
Income taxes, net			$ (32,849)
Other assets			(11,844)
Other liabilities			24,723
Net cash provided by operating activities			1,256,797
ASU 2014-09 [Member] | Difference between revenue guidance in effect before and after Topic 606 [Member]
Balance Sheets
Other current assets	$ (9,358)		(9,358)		9,002
Total current assets	(9,358)		(9,358)
Other assets	(190,276)		(190,276)		179,578
Total assets	(199,634)		(199,634)
Accounts payable and accrued expenses	(3,110)		(3,110)
Other current liabilities	18,636		18,636		(16,215)
Total current liabilities	15,526		15,526
Other liabilities	67,996		67,996		(63,051)
Total liabilities	83,522		83,522
Accumulated other comprehensive loss	7,056		7,056		(1,930)
Retained earnings	(290,212)		(290,212)		$ 269,776
Total stockholders' equity	(283,156)		(283,156)
Total liabilities and stockholders' equity	(199,634)		(199,634)
Statements of Operations
Revenues	(3,186)		(9,543)
Sales and marketing	5,202		12,880
Total costs and operating expenses	5,202		12,880
Income from operations	(8,388)		(22,423)
Income before income taxes	(8,388)		(22,423)
Income tax expense	(533)		1,987
Net income	$ (8,921)		$ (20,436)
Basic EPS (in dollars per share)	$ (0.11)		$ (0.26)
Diluted EPS (in dollars per share)	$ (0.11)		$ (0.25)
Statements of Cash Flow
Income taxes, net			$ 27
Other assets			10,565
Other liabilities			9,844
Net cash provided by operating activities			$ 0