Basis of Presentation and Significant Accounting Policies - Effects of Adopting Topic 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Balance Sheets
Other current assets	$ 249,846		$ 249,846		$ 241,029	$ 232,027
Total current assets	1,850,825		1,850,825			2,249,128
Other assets	525,961		525,961		421,328	241,750
Total assets	20,070,288		20,070,288			18,691,457
Accounts payable and accrued expenses	710,584		710,584			719,257
Other current liabilities	142,312		142,312		143,699	159,914
Total current liabilities	1,433,620		1,433,620			1,242,734
Other liabilities	633,450		633,450		598,659	661,710
Total liabilities	13,145,761		13,145,761			11,841,667
Accumulated other comprehensive loss	(877,994)		(877,994)		(787,119)	(785,189)
Retained earnings	655,101		655,101		522,465	252,689
Total stockholders' equity	6,924,527		6,924,527			6,849,790
Total liabilities and stockholders' equity	20,070,288		20,070,288			$ 18,691,457
Statements of Operations
Revenues	1,261,943	$ 1,066,421	2,477,820	$ 2,015,946
Sales and marketing	154,202	141,566	313,978	270,493
Total costs and operating expenses	1,046,905	881,526	2,036,907	1,663,838
Income from operations	215,038	184,895	440,913	352,108
Income before income taxes	73,974		153,627
Income tax expense	(6,356)	(9,325)	(23,115)	(22,718)
Net income (loss)	$ 67,618	$ 45,805	$ 130,512	$ 87,867
Basic EPS (in dollars per share)	$ 0.85	$ 0.59	$ 1.64	$ 1.17
Diluted EPS (in dollars per share)	$ 0.85	$ 0.58	$ 1.64	$ 1.16
Statements of Cash Flow
Income taxes, net			$ (22,866)	$ (33,927)
Other assets			3,536	20,285
Other liabilities			24,288	12,189
Net cash provided by operating activities			839,635	$ 553,844
Calculated under revenue guidance in effect before Topic 606 [Member]
Balance Sheets
Other current assets	$ 240,171		240,171
Total current assets	1,841,150		1,841,150
Other assets	342,118		342,118
Total assets	19,876,770		19,876,770
Accounts payable and accrued expenses	709,015		709,015
Other current liabilities	157,900		157,900
Total current liabilities	1,447,639		1,447,639
Other liabilities	701,488		701,488
Total liabilities	13,227,818		13,227,818
Accumulated other comprehensive loss	(872,278)		(872,278)
Retained earnings	373,810		373,810
Total stockholders' equity	6,648,952		6,648,952
Total liabilities and stockholders' equity	19,876,770		19,876,770
Statements of Operations
Revenues	1,259,422		2,471,463
Sales and marketing	158,578		321,656
Total costs and operating expenses	1,051,281		2,044,585
Income from operations	208,141		426,878
Income before income taxes	67,077		139,592
Income tax expense	(5,210)		(20,595)
Net income (loss)	$ 61,867		$ 118,997
Basic EPS (in dollars per share)	$ 0.78		$ 1.50
Diluted EPS (in dollars per share)	$ 0.78		$ 1.49
Statements of Cash Flow
Income taxes, net			$ (21,158)
Other assets			5,045
Other liabilities			32,586
Net cash provided by operating activities			839,635
ASU 2014-09 [Member] | Difference between revenue guidance in effect before and after Topic 606 [Member]
Balance Sheets
Other current assets	$ (9,675)		(9,675)		9,002
Total current assets	(9,675)		(9,675)
Other assets	(183,843)		(183,843)		179,578
Total assets	(193,518)		(193,518)
Accounts payable and accrued expenses	(1,569)		(1,569)
Other current liabilities	15,588		15,588		(16,215)
Total current liabilities	14,019		14,019
Other liabilities	68,038		68,038		(63,051)
Total liabilities	82,057		82,057
Accumulated other comprehensive loss	5,716		5,716		(1,930)
Retained earnings	(281,291)		(281,291)		$ 269,800
Total stockholders' equity	(275,575)		(275,575)
Total liabilities and stockholders' equity	(193,518)		(193,518)
Statements of Operations
Revenues	(2,521)		(6,357)
Sales and marketing	4,376		7,678
Total costs and operating expenses	4,376		7,678
Income from operations	(6,897)		(14,035)
Income before income taxes	(6,897)		(14,035)
Income tax expense	1,146		2,520
Net income (loss)	$ (5,751)		$ (11,515)
Basic EPS (in dollars per share)	$ (0.07)		$ (0.14)
Diluted EPS (in dollars per share)	$ (0.07)		$ (0.15)
Statements of Cash Flow
Income taxes, net			$ 1,708
Other assets			1,509
Other liabilities			8,298
Net cash provided by operating activities			$ 0