Treasury Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			1 Months Ended		2 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2012	Dec. 31, 2011	Jan. 31, 2012
Equity, Class of Treasury Stock [Line Items]
Total consideration of stock purchase	$ 19	$ 15	$ 11
Class A Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares purchased (in shares)	141,257	150,258	178,749
Treasury Stock, Shares, Retired (in shares)	141,257	150,258
Class A Common Stock [Member] | Income Tax Withholding [Member]
Equity, Class of Treasury Stock [Line Items]
Total consideration of stock purchase	19	15	9
Shares withheld in payment of income tax withholding owed by employees (in shares)	141,257	150,258	129,417
Class A Common Stock [Member] | Shareholder [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares purchased (in shares)				49,332	700,668	750,000
Price per share paid (in dollars per share)					$ 55.18
Total consideration of stock purchase					$ 41