CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Dividends on common shares (in dollars per share)	$ 1.9	$ 1.78	$ 1.67
Per share par value (in dollars per share)	$ 5	$ 5	$ 5