Debt and Credit Arrangements - Notes Interest Accretion Schedule (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument
Interest accretion of convertible notes discount		$ 9.1	$ 12.8	$ 12.5
Interest expense, debt		15.9	9.3	5.6
Total loss on extinguishment of debt		0.0	4.9	0.0
Financing costs amortization		1.3	1.3	1.3
Convertible Debt | Convertible Notes, due 2024
Debt Instrument
Interest accretion of convertible notes discount		7.2	1.1
Debt instrument, interest expense		2.6	0.4
Interest expense, debt		9.8	1.5
Financing costs amortization		0.6	0.1
Convertible Debt | Convertible Notes, due 2018
Debt Instrument
Interest accretion of convertible notes discount		1.9	11.8	12.5
Debt instrument, interest expense		1.0	4.3	5.0
Interest expense, debt		2.9	16.1	17.5
Loss on extinguishment of debt, bond cost portion		0.0	4.3	0.0
Write off of unamortized debt issuance costs		0.0	0.4	0.0
Total loss on extinguishment of debt	$ 4.9	0.0	4.7	0.0
Financing costs amortization		$ 0.1	$ 0.6	$ 0.7