CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Common stock issuance cost, less than	$ 1	$ 1	$ 1
Dividends per share of common stock (in dollars per share)	$ 3.93	$ 3.48	$ 3.08