Financing Activities (Long-term Debt and Equity Securities) (Details) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Feb. 26, 2015 USD ($) Integer	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($)
At-the-Market Stock Offering Program (Numeric) [Abstract]
Net proceeds from issuance of common stock			$ 83	$ 1,017
At The Market Stock Offering Program [Member]
At-the-Market Stock Offering Program (Numeric) [Abstract]
Number of distribution agreements | Integer	2
Aggregate sales price of common stock based on two separate equity distribution agreements	$ 500
Shares of common stock issued during the period | shares		421,700	421,700
Average price per common share | $ / shares		$ 33.73	$ 33.73
Net proceeds from issuance of common stock		$ 14	$ 14