Quarterly financial information (unaudited) - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended							12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Extinguishment of Debt [Line Items]
Pretax losses on debt extinguishment								$ 4.0	$ 0.0	$ 0.0
Restructuring charges	$ 13.1	$ 31.9	$ 10.2	$ 26.9	$ 48.5	$ 17.4	$ 52.6	82.1	$ 118.5
6.25% senior notes due August 15, 2017 | Senior Notes
Extinguishment of Debt [Line Items]
Pretax losses on debt extinguishment								$ 4.0