12. Earnings per Share

Total outstanding shares as of December 31, 2017, 2016 and 2015 were 106.7 million, 107.9 million and 105.6 million respectively. Under our stock repurchase program, for the years ended December 31, 2017 and 2015, we repurchased and retired 3.3 million and 7.7 million shares of common stock, respectively. No shares were repurchased during the year ended December 31, 2016. For each of the years ended December 31, 2017, 2016 and 2015, we acquired 0.7 million shares, related to tax withholding for share-based compensation. The calculation of basic earnings per common share is based on the weighted average number of common shares outstanding in the year, or period, over which they were outstanding. Our calculation of diluted earnings per common share assumes that any common shares outstanding were increased by shares that would be issued upon exercise of those stock units for which the average market price for the period exceeds the exercise price less the shares that could have been purchased by the Company with the related proceeds, including compensation expense measured but not yet recognized.



(in millions)	2017	2016	2015
Weighted-average number of common shares outstanding - basic	108.1	107.0	108.8
Stock options	1.3	0.8	0.2
Stock-settled stock appreciation rights	—	—	0.3
Restricted stock units	1.5	1.4	1.3
Adjusted weighted-average shares and assumed conversions - diluted	110.9	109.2	110.6

Awards of potentially dilutive shares of common stock, which have exercise prices that were higher than the average market price during the period, are not included in the computation of dilutive earnings per share as their effect would have been anti-dilutive. For the years ended December 31, 2017, 2016 and 2015, these shares were approximately 3.1 million, 3.6 million and 5.5 million, respectively.