Stockholders' Equity and Compensation Arrangements Repurchase Program (Details) - Common Class A [Member] $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) shares	Jun. 30, 2017 USD ($) shares
Employee Stock Purchase Plan [Line Items]
Stock Repurchase Program, Authorized Amount | $	$ 700.0	$ 700.0
Stock Repurchased During Period, Shares | shares	7,185,453	14,522,134
Stock Repurchased During Period, Value | $	$ 24.8	$ 50.9
Stock Repurchase Program, Remaining Number of Shares Authorized to be Repurchased | shares	144,300,000	144,300,000