Net Income per common share - Additonal Information (Details) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	1	14	15
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Basic Earnings Per Share	$ 0.03
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share	$ 0.02