Debt (Convertible Debenture Terms) (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares
Debt Disclosure [Abstract]
Outstanding principal amount	$ 287
Unamortized debt discount	93
Net debt carrying amount	191
Equity Component carrying amount	$ 104
Conversion rate (shares of common stock per $1,000 of principal amount)	13.8474
Effective conversion price (per $1,000 of principal amount) per share | $ / shares	$ 72.2151