Net Income per Share (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Amount authorized under Stock Repurchase Program	$ 1,500,000,000
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from EPS		400
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from EPS	1,500	200
Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of shares repurchased	18
Average purchase price (in dollars per share)	$ 53.82
Available for repurchase under the Stock Repurchase Program	$ 558,400,000

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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