Derivative Instruments - Balance Sheet Offseting (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Cash collateral
Cash collateral in margin account	$ 22.4	$ 42.3
Offsetting Derivative Assets
Gross amount recognized on the balance sheet	37.6	9.9
Gross amount not offset on the balance sheet	(4.2)	(3.0)
Net amount	33.4	6.9
Offsetting Derivative Liabilities
Gross amount recognized on the balance sheet	19.9	59.0
Gross amount not offset on the balance sheet	(4.2)	(22.5)
Net amount	15.7	36.5
Cash collateral posted	0.0	19.5
Credit Risk Related Contingent Features
Aggregate fair value of derivative instruments with credit risk-related contingent features that were in a liability position	3.1	23.8
Collateral that would have been required	$ 1.3	$ 18.0

The aggregate fair value of assets needed to immediately settle the derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

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Fair value of liability associated with financial asset or other 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, and obligation to return cash collateral under master netting arrangements.

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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