Computation of Earnings Per Share (Details) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Tax act, income tax benefit (in dollars per share)	$ 0.66
Common Stock Equivalents (CSEs)
Class of Stock [Line Items]
Stock repurchased during period (in shares)	0.4	0.6	6.2