Regulatory Assets and Liabilities and Regulatory Matters - Narrative (Details) - USD ($) $ in Thousands						1 Months Ended	6 Months Ended	12 Months Ended
	Apr. 20, 2017	Jan. 19, 2017	Oct. 14, 2016	Apr. 01, 2016	Apr. 01, 2015	Mar. 31, 2016	Mar. 31, 2017	Sep. 30, 2014	Sep. 30, 2016
Pennsylvania PUC
Regulatory Assets
Maximum period post petition to file general rate filing (in years)							5 years
Pennsylvania PUC | PNG
Regulatory Assets
Requested operating revenue increase		$ 21,700
Requested rate change, review process period		9 months
Pennsylvania PUC | PNG | Subsequent Event
Regulatory Assets
Distribution system improvement charge, percent of amount billed to customers	7.50%
Pennsylvania PUC | CPG
Regulatory Assets
Operating revenue increase			$ 27,000
Pennsylvania PUC | CPG | Subsequent Event
Regulatory Assets
Distribution system improvement charge, percent of amount billed to customers	7.50%
Pennsylvania PUC | Maximum
Regulatory Assets
Distribution system improvement charge, percent of amount billed to customers						5.00%	5.00%	0.00%
Pennsylvania PUC | Maximum | PNG
Regulatory Assets
Distribution system improvement charge, percent of amount billed to customers					0.00%	10.00%
Pennsylvania PUC | Maximum | CPG
Regulatory Assets
Distribution system improvement charge, percent of amount billed to customers				0.00%		10.00%
Gas Utility
Regulatory Assets
Fair value of unrealized gains (losses)						$ (1,900)	$ 1,973		$ 4,263