Derivative Instruments and Hedging Activities - Balance Sheet Location and Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Jun. 30, 2015
Derivative Assets
Derivative assets:
Derivative assets	$ 5,718	$ 1,979
Gross amounts offset in the balance sheet	(225)	(583)
Total derivative assets - net	5,493	1,396
Derivative Liabilities
Derivative liabilities:
Derivative liabilities	(732)	(4,995)
Gross amounts offset in the balance sheet	225	583
Total derivative liabilities - net	(507)	(4,412)
Derivatives Subject to PGC and DS Mechanisms | Commodity contracts | Derivative Assets
Derivative assets:
Derivative assets	5,718	1,943
Derivatives Subject to PGC and DS Mechanisms | Commodity contracts | Derivative Liabilities
Derivative liabilities:
Derivative liabilities	(593)	(4,807)
Derivatives Not Subject To PGC And DS Mechanisms | Commodity contracts | Derivative Assets
Derivative assets:
Derivative assets	0	36
Derivatives Not Subject To PGC And DS Mechanisms | Commodity contracts | Derivative Liabilities
Derivative liabilities:
Derivative liabilities	$ (139)	$ (188)