Derivative Financial Instruments (Offsetting of Derivative Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gross Amount of Recognized Liabilities, Derivative liabilities	$ (80.0)	$ (68.8)
Gross Amount Offset in the Consolidated Balance Sheet, Derivative liabilities	0.0	0.0
Net Amount Presented in the Consolidated Balance Sheet, Derivative liabilities	(80.0)	(68.8)
Derivative Financial Instruments, Derivative liabilities	20.0	30.9
Cash Collateral Pledged/ (Received), Derivative liabilities	21.6	5.0
Net Amount	(38.4)	$ (32.9)
Variable margin balances	$ 19.7

Margin Deposit Liabilities

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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value, after effects of master netting arrangements, deduction of liabilities not subject to a master netting arrangement and elected not to be offset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, offset against the right to receive collateral.

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Fair value of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, not subject to a master netting arrangement, deducted from derivative liabilities.

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