Stockholders' equity - Changes in Preferred Stock, Common Stock and Treasury Stock (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Preferred Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	0.0	0.0	2.6
Common stock repurchases (in shares)			0.0
Stock-based compensation (in shares)	0.0	0.0	0.0
Preferred stock conversion (in shares)			(2.6)
Ending Balance (in shares)	0.0	0.0	0.0
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	52.6	52.4	45.1
Common stock repurchases (in shares)			0.0
Stock-based compensation (in shares)	0.2	0.2	0.4
Preferred stock conversion (in shares)			6.9
Ending Balance (in shares)	52.8	52.6	52.4
Treasury Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance (in shares)	2.7	2.7	1.1
Common stock repurchases (in shares)			1.6
Stock-based compensation (in shares)	0.0	0.0	0.0
Preferred stock conversion (in shares)			0.0
Ending Balance (in shares)	2.7	2.7	2.7