Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Common Stock, par value (in dollars per share)	$ 0	$ 0
Common Stock, shares authorized (in shares)	450,000,000	450,000,000
Common Stock, shares outstanding (in shares)	240,000,000	252,000,000