Related Party Transactions - Summary of Activity Related to Agreements with Related Party (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019		Sep. 30, 2020	Sep. 30, 2019
Related Party Transaction [Line Items]
Earnings from equity investments	$ 3.6	$ 0.5	[1]	$ 7.2	$ 0.5	[1]
Distributions received from equity investments				6.8
LM4
Related Party Transaction [Line Items]
Processing fee incurred	6.8	0.9		18.2	0.9
Earnings from equity investments	3.6	$ 0.5		7.2	$ 0.5
Distributions received from equity investments	$ 3.0			$ 6.8

[1] The financial information presented in this report has been retrospectively adjusted for the acquisition of Hess Infrastructure Partners LP.