Derivative instruments: Interest rate options (Details) - Interest rate options	3 Months Ended
	Dec. 31, 2011 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative instruments and hedging activities
Purchased amount of derivative instrument	$ 100,000,000
Number of nuclear units expected to be financed | item	2
Derivative notional amount expired		$ 1,788,502,000
Derivative remaining notional amount		390,702,000
Cash settlement value, if swap rates are at or below the specified fixed rate on the expiration date		$ 0
Weighted average fixed rate (as a percent)		3.95%
Notional amount of derivatives, expired without value		$ 470,625,000
Fair value of assets		1,010,000	$ 4,371,000
Funds posted as collateral of the counterparties		0	$ 0
Notional Dollar Amount		$ 390,702,000
LIBOR
Derivative instruments and hedging activities
Variable rate basis	LIBOR
Basis spread (as a percent)		1.51%
2016
Derivative instruments and hedging activities
Notional Dollar Amount		$ 310,533,000
2017
Derivative instruments and hedging activities
Notional Dollar Amount		80,169,000
Minimum
Derivative instruments and hedging activities
Collateral thresholds range		0
Maximum
Derivative instruments and hedging activities
Collateral thresholds range		10,000,000
Department of Energy-guaranteed loan
Derivative instruments and hedging activities
Hedged amount of expected debt	$ 2,200,000,000	$ 2,200,000,000