Derivative instruments: - Interest rate options (Details) - Interest rate options	3 Months Ended
	Dec. 31, 2011 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Derivative instruments and hedging activities
Number of derivative instruments originally purchased | item	17
Purchased amount of derivative instrument	$ 100,000,000
Number of nuclear units expected to be financed | item	2
Number of derivative instruments that have expired | item		16
Derivative notional amount expired		$ 2,099,035,000
Derivative remaining notional amount		$ 80,169,000
Weighted average fixed rate (as a percent)		3.88%
Notional amount of derivatives, expired without value		$ 310,533,000
Fair value of assets		0	$ 1,010,000
Funds posted as collateral of the counterparties		$ 0
LIBOR
Derivative instruments and hedging activities
Variable rate basis	LIBOR
Basis spread (as a percent)		1.39%
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