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Subsequent Events	9 Months Ended
Subsequent Events	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Repurchase of Class A Common Stock Since September 30, 2018, we repurchased 423,863 shares at a weighted average price of $76.85 per share and, as of October 26, 2018, under our existing share repurchase authorization, $18.3 million remained available for share repurchases. In addition, on October 22, 2018, the Board of Directors increased our repurchase authorization by an additional $250 million, bringing our total repurchase authorization to $268 million.