Summary of Significant Accounting Policies (Narrative) (Details) $ / shares in Units, shares in Thousands			12 Months Ended
	Oct. 21, 2020 USD ($) $ / shares shares	Jan. 01, 2020	Dec. 31, 2020 USD ($) customer $ / shares shares	Dec. 31, 2019 USD ($) customer shares	Dec. 31, 2018 USD ($) shares	Sep. 16, 2020 USD ($) $ / shares
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Advertising costs			$ 246,900,000	$ 180,300,000	$ 196,000,000.0
Foreign currency transaction gains (losses)			(4,000,000.0)	11,100,000	(6,800,000)
Accounts receivable contractually due in excess of one-year			$ 267,500,000	$ 273,000,000.0
Antidilutive securities excluded from EPS | shares			300	600
Repurchase amount (up to)			$ 1,500,000,000
Average purchase (in dollars per share) | $ / shares			$ 23.91
Available for repurchase under program			$ 135,300,000
Aggregate cost	$ 250,600,000		356,701,000	$ 70,598,000	$ 283,143,000
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member
Decrease to retained earnings			$ (1,847,451,000)	$ (1,609,428,000)
Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings				$ 2,000,000.0
Restricted Stock Units (RSUs) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Antidilutive securities excluded from EPS | shares			300	1,300	1,000
Accounts Receivable [Member] | Customer Concentration Risk [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of customers | customer			2	2
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer One [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk, percentage			18.00%	16.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer Two [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk, percentage			10.00%	10.00%
Common Class A [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock, voting rights			one
Common stock, conversion basis			one
Stockholders entitled election of Board of Directors			25.00%
Repurchase amount (up to)						$ 250,000,000
Share repurchases | shares	10,800		14,785	1,302	5,386
Average purchase (in dollars per share) | $ / shares	$ 23.20
Additional percentage						2.00%
Common Class B [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock, voting rights			ten
Common stock, conversion basis			one
Share repurchases | shares			0	0	0
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Period of related affiliation arrangement (up to)			10 years
Maximum [Member] | Common Class A [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Share price (in dollars per share) | $ / shares						$ 26.50
Minimum [Member] | Common Class A [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Share price (in dollars per share) | $ / shares						$ 22.50
2011 Non-Employee Director Plan [Member] | Restricted Stock Units (RSUs) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Vesting percentage			100.00%