Common Stock (Details) - Income Tax Withholding [Member] - Class A Common Stock [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Equity, Class of Treasury Stock [Line Items]
Shares withheld in payment of income tax withholding owed by employees (in shares)	4,636	7,206	119,788	122,642
Total consideration of stock purchase	$ 1	$ 1	$ 24	$ 18