Earnings per Share Share Repurchase (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount | $	$ 300
Stock Repurchased and Retired During Period Weighted Average Price | $ / shares	$ 72.35
Stock Repurchased and Retired During Period Weighted Average Basic Shares | shares	51,106
Stock Repurchase Program, Remaining Authorized Repurchase Amount | $	$ 243
Class A Common Stock, $0.01 par value, one vote per share; 180,000,000 shares authorized at March 31, 2018 and December 31, 2017; 74,924,202 and 74,808,305 shares issued and 61,229,743 and 61,946,078 shares outstanding (net of 13,694,459 and 12,862,227 shares in treasury) on March 31, 2018 and December 31, 2017, respectively
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased During Period, Shares | shares	792,921