Fair Value Measurements Level 3 (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loss on Series B Preferred Stock dividend make-whole provisions	$ (67.7)	$ 0.0	$ (124.8)	$ 0.0
Derivative Financial Instruments, Assets [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance net asset (liability)	(7.9)	(77.0)	79.3	0.0
Net asset (liability) at September 30	77.4	(78.8)	77.4	(78.8)
Derivative Financial Instruments, Assets [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value, Measurements, Recurring [Member] \| Interest Charges [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	28.2	(1.8)	(1.9)	(78.8)
Total unrealized gains (losses) relating to assets and liabilities still on the consolidated balance sheet at September 30:	28.2	(1.8)	(1.9)	(78.8)
Derivative Financial Instruments, Liabilities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Settlements	124.8	0.0	124.8	0.0
Derivative Financial Instruments, Liabilities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value, Measurements, Recurring [Member] \| Loss On Series B Preferred Stock Dividend Make-Whole Provisions [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	$ (67.7)	$ 0.0	$ (124.8)	$ 0.0

Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
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Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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Amount of gain (loss) recognized in the income statement of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative liability, measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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