Derivative instruments - Interest rate options (Details) - Interest rate options	3 Months Ended
	Dec. 31, 2011 USD ($) item	Mar. 31, 2017 USD ($)
Derivative instruments and hedging activities
Number of derivative instruments originally purchased | item	17
Purchased amount of derivative instrument	$ 100,000,000
Notional Dollar Amount	$ 2,200,000,000
Number of nuclear units expected to be financed | item	2
Derivative notional value expired		$ 80,169,000