Debt (details) - Repayment - USD ($) $ in Millions			12 Months Ended
	Dec. 01, 2015	Mar. 15, 2013	Dec. 31, 2015	Dec. 31, 2013	Dec. 31, 2014
Debt Instrument
Debt, principal amount repaid			$ 400	$ 500
5.50% Senior notes due December 1, 2015 [Member]
Debt Instrument
Debt, principal amount repaid	$ 400
Interest rate (percent)	5.50%				5.50%
5.00% Senior notes due March 15, 2013 [Member]
Debt Instrument
Debt, principal amount repaid		$ 500
Interest rate (percent)		5.00%