Other Commitments and Contingencies - Additional Information (Detail) gal in Millions, $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2013	Dec. 31, 2017 USD ($) gal
Subsequent Event [Line Items]
Intercompany Agreements, Description		3
Number Of Years To Purchase Propane Under Contract	6 years
Annual Estimate Of Volume Of Propane To Be Purchased | gal		2.7
Debt Service Coverage Ratio		1.25
Time to cure ratio		5 days
Ratio based on average number of prior quarters		6
Funds from operations interest coverage ratio minimum times		2
Total debt to capital maximum		65
Number Of Years to Collect Benefits		20 years
Total purchase obligations for 2018		$ 152.9
Total purchase obligations for 2019 - 2020		122.8
Total purchase obligations for 2021 - 2022		44.6
Total purchase obligations thereafter		149.6
Maximum authorized liability under such guarantees and letters of credit		95.0
Aggregate guaranteed amount		72.0
Amount of letter of credit to our current primary insurance company		$ 5.0