Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended			12 Months Ended
	Feb. 29, 2016	Jan. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Stockholders Equity [Abstract]
Common stock, shares authorized (in shares)			1,000,000,000	1,000,000,000	1,000,000,000
Common stock, par value (in dollars per share)			$ 0.10	$ 0.10	$ 0.10
Preferred Stock, Shares Authorized				4,500,000
Preferred Stock, Par or Stated Value Per Share				$ 1.00
Net proceeds from offering				$ 1,469
Common Stock [Member]
Stockholders Equity [Abstract]
Common stock, shares issued				103,000,000	7,000,000
Common Stock Offering [Member]
Stockholders Equity [Abstract]
Common stock, shares issued	79,000,000
Net proceeds from offering	$ 1,500
Common Stock Offering [Member] | Underwriters [Member]
Stockholders Equity [Abstract]
Common stock, shares issued	10,000,000
Equity Issued in Business Combination [Member] | Common Stock [Member] | Powder River Basin [Member]
Stockholders Equity [Abstract]
Equity issued for acquisition			7,000,000
Equity Issued in Business Combination [Member] | Common Stock [Member] | STACK [Member]
Stockholders Equity [Abstract]
Equity issued for acquisition		23,000,000