Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative assets:
Derivative assets	$ 613	$ 3,526	[1]
Derivative liabilities:
Derivative liabilities	(5,540)	(677)	[1]
Commodity Contracts | Derivatives Subject to Utility Rate Regulation
Derivative assets:
Derivative assets	613	3,458	[1]
Derivative liabilities:
Derivative liabilities	(5,168)	(677)	[1]
Commodity Contracts | Derivatives Not Designated as Hedging Instruments
Derivative assets:
Derivative assets	0	68	[1]
Derivative liabilities:
Derivative liabilities	$ (372)	$ 0	[1]

[1] Certain immaterial amounts have been revised to correct the classification of derivatives.