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Debt 0.75% Senior convertible notes due 2022 (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Current Fiscal Year End Date	--12-31
Principal amount	$ 400.0	$ 400.0
Unamortized debt discount	(57.8)	(64.4)
Unamortized debt issuance costs	(7.2)	(8.0)
Net carrying amount of senior convertible notes	335.0	327.6
Fair value of outstanding notes	465.8	381.3
Amount by which the notes' if-converted value exceeds their principal amount	$ 55.4	$ 0.0

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Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock.

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Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Convertible Notes Payable, excluding current portion. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Face (par) amount of debt instrument at time of issuance.

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-SubTopic 30
-Section 55
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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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