Fair Value Measurements - Summary of Net Changes in Fair Value of the Company's Level 3 Financial Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Summary of net changes in fair value of the company's level 3 financial assets and liabilities
Realized (gain) loss on derivative instruments	$ 11.3	$ 10.3	$ 49.8	$ 19.4
Unrealized gain (loss) on derivative instruments	(1.2)	(12.2)	30.4	(14.8)
Level 3
Summary of net changes in fair value of the company's level 3 financial assets and liabilities
Fair value at January 1,			(6.3)	19.8
Realized (gain) loss on derivative instruments			(19.4)	(34.2)
Unrealized gain (loss) on derivative instruments			30.4	(14.8)
Settlements			(19.4)	(34.2)
Fair value at June 30,	$ 24.1	$ 5.0	24.1	5.0
Total gain (loss) included in net loss attributable to changes in unrealized gain relating to financial assets and liabilities held as of June 30,			$ 30.4	$ (14.8)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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Amount of settlement of financial instrument classified as an 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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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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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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Amount of unrealized gain (loss) on investment.

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