Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Mar. 31, 2015
Derivative Assets
Derivative assets:
Derivative assets	$ 1,179	$ 613
Gross amounts offset in the balance sheet	(298)	(1)
Total derivative assets - net	881	612
Derivative Liabilities
Derivative liabilities:
Derivative liabilities	(3,668)	(5,540)
Gross amounts offset in the balance sheet	298	1
Total derivative liabilities - net	(3,370)	(5,539)
Derivatives Subject to PGC and DS Mechanisms | Commodity Contracts | Derivative Assets
Derivative assets:
Derivative assets	1,179	613
Derivatives Subject to PGC and DS Mechanisms | Commodity Contracts | Derivative Liabilities
Derivative liabilities:
Derivative liabilities	(3,466)	(5,168)
Derivatives Not Subject To PGC And DS Mechanisms | Commodity Contracts | Derivative Liabilities
Derivative liabilities:
Derivative liabilities	$ (202)	$ (372)