Asset impairment and facility consolidation charges (gains) - Narrative (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unusual or Infrequent Item, or Both [Line Items]
Asset impairment charges						$ 2.2	$ 4.7
Gain on sale of corporate headquarters				$ 89.9
Goodwill impairment charges			$ 332.9				15.2	$ 8.0
Hurricane Harvey
Unusual or Infrequent Item, or Both [Line Items]
Net expense impact from hurricane		$ 0.9			$ 0.9
Gross hurricane related expenses		26.9			15.8
Insurance recovery	$ 21.0	$ 5.0			$ 26.0	$ 26.0
Cofactor Reporting Unit
Unusual or Infrequent Item, or Both [Line Items]
Goodwill impairment charges							15.2
Internally produced program
Unusual or Infrequent Item, or Both [Line Items]
Asset impairment charges							$ 6.3