Summary of Significant Accounting Policies - Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2017
Minimum | Propane Distribution Mains [Member]
Useful Life of Assets	10 years
Minimum | Propane Bulk Plants And Tanks [Member]
Useful Life of Assets	10 years
Minimum | Liquefied Petroleum Gas Equipment [Member]
Useful Life of Assets	5 years
Minimum | Meters And Meter Installations [Member]
Useful Life of Assets	5 years
Minimum | Measuring And Regulating Station Equipment [Member]
Useful Life of Assets	5 years
Minimum | Natural gas processing equipment [Member]
Useful Life of Assets	20 years
Minimum | Office Furniture And Equipment [Member]
Useful Life of Assets	3 years
Minimum | Transportation Equipment [Member]
Useful Life of Assets	4 years
Minimum | Structures And Improvements [Member]
Useful Life of Assets	5 years
Maximum | Propane Distribution Mains [Member]
Useful Life of Assets	37 years
Maximum | Propane Bulk Plants And Tanks [Member]
Useful Life of Assets	40 years
Maximum | Liquefied Petroleum Gas Equipment [Member]
Useful Life of Assets	33 years
Maximum | Meters And Meter Installations [Member]
Useful Life of Assets	33 years
Maximum | Measuring And Regulating Station Equipment [Member]
Useful Life of Assets	37 years
Maximum | Natural gas pipelines [Member]
Useful Life of Assets	45 years
Maximum | Natural gas processing equipment [Member]
Useful Life of Assets	25 years
Maximum | Office Furniture And Equipment [Member]
Useful Life of Assets	10 years
Maximum | Transportation Equipment [Member]
Useful Life of Assets	20 years
Maximum | Structures And Improvements [Member]
Useful Life of Assets	45 years