12. Earnings per Share

Total outstanding shares as of March 31, 2018 and 2017 were 107.8 million and 109.5 million, respectively. Under our stock repurchase program, for the three months ended March 31, 2018 we repurchased and retired 0.8 million shares. No shares were repurchased during the three months ended March 31, 2017. For each of the three months ended March 31, 2018 and 2017, we acquired 0.6 million and 0.7 million shares, respectively related to tax withholding for share-based compensation. The calculation of basic earnings per share of common stock is based on the weighted average number of shares of common stock outstanding in the year, or period, over which they were outstanding. Our calculation of diluted earnings per share of common stock assumes that any shares of common stock 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.



	Three Months Ended March 31,
(in millions)	2018	2017
Weighted-average number of shares of common stock outstanding - basic	106.8	108.3
Stock options	1.2	1.5
Restricted stock units	2.0	2.6
Adjusted weighted-average shares and assumed conversions — diluted	110.0	112.4

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 three months ended March 31, 2018 and 2017, the number of anti-dilutive shares were approximately 2.8 million and 2.3 million, respectively.