Debt and Credit Arrangements - Notes Interest Accretion Schedule (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Instrument
Interest accretion of convertible notes discount			$ 5.0	$ 6.6
Financing costs amortization	$ 0.4	$ 0.4	0.7	0.7
Convertible Debt | Convertible Notes, due 2024
Debt Instrument
Interest accretion of convertible notes discount	1.8		3.5
Debt instrument, interest expense	0.7		1.3
Interest expense	2.5		4.8
Financing costs amortization	0.1		0.3
Convertible Debt | Convertible Notes, Due 2018
Debt Instrument
Interest accretion of convertible notes discount	0.7	3.3	1.5	6.6
Debt instrument, interest expense	0.3	1.2	0.9	2.5
Interest expense	1.0	4.5	2.4	9.1
Financing costs amortization	$ 0.1	$ 0.2	$ 0.1	$ 0.4