Other Commitments and Contingencies - Additional Information (Detail) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2019	Jun. 30, 2019 USD ($)
Other Commitments [Line Items]
Debt Service Coverage Ratio		1.25
Debt Service Coverage Ratio Measurement Period		12
Maximum Days To Make Default Good		5 days
Ratio based on average number of prior quarters		6
Ratios based on average of the prior quarters	1 year 6 months
Funds from operations interest coverage ratio minimum times		2
Total debt to capital maximum		0.65
Number Of Years For Power Purchase Agreement		20 years
Contract Duration		20 years
Aggregate guaranteed amount		$ 68.1
Draws on letters of credit		$ 7.0
Florida Natural Gas Distribution and Eight Flags [Member]
Other Commitments [Line Items]
Number of Years for Asset Management Agreement		10 years
Delmarva [Member]
Other Commitments [Line Items]
Number of Years for Asset Management Agreement		3 years