Nature of Business and Summary of Significant Accounting Policies - Effects of Adopting Topic 606 (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	Jan. 01, 2018	Dec. 31, 2015
Balance Sheets
Accounts receivable, net	$ 630,119				$ 576,313				$ 630,119	$ 576,313		$ 576,313
Other current assets	274,857				232,027				274,857	232,027		241,029
Total current assets	1,515,682				2,249,128				1,515,682	2,249,128
Other assets	533,252				241,750				533,252	241,750		421,328
Total assets	20,244,638				18,691,457				20,244,638	18,691,457
Accounts payable and accrued expenses	756,692				719,257				756,692	719,257
Other current liabilities	126,995				159,914				126,995	159,914		143,699
Total current liabilities	1,515,071				1,242,734				1,515,071	1,242,734
Other liabilities	629,763				661,710				629,763	661,710		598,659
Total liabilities	13,025,359				11,841,667				13,025,359	11,841,667
Accumulated other comprehensive loss	(945,702)				(785,189)				(945,702)	(785,189)		(787,119)
Retained earnings	889,948				252,689				889,948	252,689		522,465
Total stockholders' equity	7,219,279				6,849,790				7,219,279	6,849,790	$ 4,365,829		$ 2,745,386
Total liabilities and stockholders' equity	20,244,638				18,691,457				20,244,638	18,691,457
Statements of Operations
Revenues	1,310,083	$ 1,283,751	$ 1,261,943	$ 1,215,877	1,200,221	$ 1,152,261	$ 1,066,421	$ 949,525	5,071,654	4,368,428	3,611,989
Sales and marketing									633,702	581,724	438,742
Total costs and operating expenses									4,094,271	3,559,414	2,993,250
Income from operations									977,383	809,014	618,739
Income before income taxes									433,038	286,832	159,859
Income tax expense									(67,679)	(53,850)	(45,451)
Net income from continuing operations									365,359	232,982	114,408
Net income	$ 110,022	$ 124,825	$ 67,618	$ 62,894	$ 65,215	$ 79,900	$ 45,805	$ 42,062	$ 365,359	$ 232,982	$ 126,800
Basic EPS (in dollars per share)	$ 1.37	$ 1.56	$ 0.85	$ 0.79	$ 0.83	$ 1.02	$ 0.59	$ 0.58	$ 4.58	$ 3.03	$ 1.81
Diluted EPS (in dollars per share)	$ 1.36	$ 1.55	$ 0.85	$ 0.79	$ 0.82	$ 1.02	$ 0.58	$ 0.57	$ 4.56	$ 3.00	$ 1.79
Statements of Cash Flow
Net income	$ 110,022	$ 124,825	$ 67,618	$ 62,894	$ 65,215	$ 79,900	$ 45,805	$ 42,062	$ 365,359	$ 232,982	$ 126,800
Accounts receivable									(52,931)	(161,774)	(100,230)
Income taxes, net									(10,670)	(34,936)	29,020
Other assets									(47,635)	20,180	(72,831)
Other liabilities									31,725	5,708	(50,558)
Net cash provided by operating activities									1,815,426	$ 1,439,233	$ 1,019,353
Calculated under revenue guidance in effect before Topic 606
Balance Sheets
Accounts receivable, net	627,733								627,733
Other current assets	265,027								265,027
Total current assets	1,503,466								1,503,466
Other assets	340,946								340,946
Total assets	20,040,116								20,040,116
Accounts payable and accrued expenses	753,489								753,489
Other current liabilities	144,911								144,911
Total current liabilities	1,529,784								1,529,784
Other liabilities	703,177								703,177
Total liabilities	13,113,486								13,113,486
Accumulated other comprehensive loss	(937,856)								(937,856)
Retained earnings	589,453								589,453
Total stockholders' equity	6,926,630								6,926,630
Total liabilities and stockholders' equity	20,040,116								20,040,116
Statements of Operations
Revenues									5,056,239
Sales and marketing									653,928
Total costs and operating expenses									4,114,497
Income from operations									941,742
Income before income taxes									397,397
Income tax expense									(62,757)
Net income from continuing operations									334,640
Net income									$ 334,640
Basic EPS (in dollars per share)									$ 4.19
Diluted EPS (in dollars per share)									$ 4.17
Statements of Cash Flow
Net income									$ 334,640
Accounts receivable									(51,518)
Income taxes, net									(12,533)
Other assets									(29,587)
Other liabilities									44,846
Net cash provided by operating activities									1,815,426
Accounting Standards Update 2014-09 | Difference between revenue guidance in effect before and after Topic 606
Balance Sheets
Accounts receivable, net	(2,386)								(2,386)
Other current assets	(9,830)								(9,830)			9,002
Total current assets	(12,216)								(12,216)
Other assets	(192,306)								(192,306)			179,578
Total assets	(204,522)								(204,522)
Accounts payable and accrued expenses	(3,203)								(3,203)
Other current liabilities	17,916								17,916			(16,215)
Total current liabilities	14,713								14,713
Other liabilities	73,414								73,414			(63,051)
Total liabilities	88,127								88,127
Accumulated other comprehensive loss	7,846								7,846			(1,930)
Retained earnings	(300,495)								(300,495)			$ 269,776
Total stockholders' equity	(292,649)								(292,649)
Total liabilities and stockholders' equity	$ (204,522)								(204,522)
Statements of Operations
Revenues									(15,415)
Sales and marketing									20,226
Total costs and operating expenses									20,226
Income from operations									(35,641)
Income before income taxes									(35,641)
Income tax expense									4,922
Net income from continuing operations									(30,719)
Net income									$ (30,719)
Basic EPS (in dollars per share)									$ (0.39)
Diluted EPS (in dollars per share)									$ (0.39)
Statements of Cash Flow
Net income									$ (30,719)
Accounts receivable									1,413
Income taxes, net									(1,863)
Other assets									18,048
Other liabilities									13,121
Net cash provided by operating activities									$ 0