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Fair Value Measurements - Changes in Fair Value of Financial Instruments Classified as Level 3 (Details) - Commodity Derivative Contracts Asset (Liability) [Member]
$ in Millions
9 Months Ended
Sep. 30, 2021
USD ($)
Changes in fair value of financial instruments classified as Level 3 in fair value hierarchy [Roll Forward]  
Balance, beginning of period $ (0.2)
New Level 3 derivative instruments 0.0
Transfers out of Level 3 0.2 [1]
Unrealized gain (loss) included in OCI (0.1)
Balance, end of period $ (0.1)
[1]

Transfers relate to long-term over-the-counter swaps for NGL products for which observable market prices became available for substantially their full term.