Related Party Transactions - Summary of Activity Related to Agreements with Related Party (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Related Party Transaction [Line Items]
Earnings from equity investments	$ 2.7	$ 2.7
Distributions received from equity investments	7.5
LM4
Related Party Transaction [Line Items]
Processing fee incurred	6.9	6.4
Earnings from equity investments	2.7	$ 2.7
Distributions received from equity investments	$ 7.5

Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

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