Earnings per share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator [Abstract]
Net Income from continuing operations		$ (3.4)	$ 20.9	$ 50.1	$ (4.9)	$ 33.9	$ 33.6	$ 4.6	$ 10.5	$ 62.7	$ 82.6	$ 62.5
Interest expense, net of tax										0.0	2.6	3.3
Amortization of debt issuance costs, net of tax										0.0	0.7	0.8
Net income, adjusted from continuing operations										62.7	85.9	66.6
Net loss from discontinued operations										0.0	0.0	(10.7)
Net income, adjusted										$ 62.7	$ 85.9	$ 55.9
Denominator [Abstract]
Weighted-average number of shares-basic (in shares)										50.1	41.8	40.2
Dilutive securities-equity awards (in shares)										1.3	1.1	1.0
Dilutive securities-convertible debt (in shares)										0.0	7.4	8.1
Weighted-average number of shares-diluted (in shares)										51.4	50.3	49.3
Net income per share from continuing operations-basic (in dollars per share)		$ (0.07)	$ 0.42	$ 1.00	$ (0.10)	$ 0.77	$ 0.81	$ 0.11	$ 0.26	$ 1.25	$ 1.98	$ 1.56
Net loss per share from discontinued operations-basic (in dollars per share)										0	0	(0.27)
Net income per share-basic (in dollars per share)										1.25	1.98	1.29
Net income per share-diluted from continuing operations (in dollars per share)		$ (0.07)	$ 0.41	$ 0.98	$ (0.10)	$ 0.67	$ 0.68	$ 0.11	$ 0.23	1.22	1.71	1.35
Net loss per share-diluted from discontinued operations (in dollars per share)										0	0	(0.22)
Net income per share-diluted (in dollars per share)	[1]									$ 1.22	$ 1.71	$ 1.13
Stock Options [Member]
Antidilutive shares excluded from computation of earnings per share [Abstract]
Antidilutive shares excluded from calculation (in shares)												1.4

[1] See "Earnings per share" footnote for details on calculation.