9. Earnings Per Common Share

Basic earnings per common share is computed by dividing net income by the weighted average number of shares of common stock outstanding during the period. Diluted earnings per common share is computed by dividing net income by the weighted average number of shares of common stock outstanding during the period, adjusted for the potential dilutive effect of our outstanding stock options, as if such options were exercised.

For the three months ended March 31, 2019, we had a net loss, and as such, all outstanding stock options and restricted stock units were excluded from our calculation of diluted (loss) earnings per share. The components of basic and diluted (loss) earnings per common share comprised the following (in millions, except per share amounts):


	Three Months Ended
	March 31,
	2019		2018
Numerator:
Net (loss) income	$	(494.6)	$	244.5
Denominator:
Weighted average outstanding shares - basic		43.7		43.3
Effect of dilutive securities(1):
Stock options, restricted stock units and employee stock purchase plan		—		0.6
Weighted average shares - diluted(2)		43.7		43.9
Net (loss) income per common share:
Basic	$	(11.32)	$	5.65
Diluted	$	(11.32)	$	5.57

Stock options and restricted stock units excluded from calculation(2)		5.3		3.9

(1)

Calculated using the treasury stock method.

(2)

The common shares underlying certain stock options and restricted stock units have been excluded from the computation of diluted earnings per share because their impact would be anti-dilutive for the three-month periods ended March 31, 2019 and March 31, 2018.