Derivative instruments - Volume activity of natural gas derivatives (Details) - Natural gas swaps item in Thousands	Mar. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Gas hedges
Derivative assets | $	$ 4,968,000	$ 15,090,000
Collateral or letters of credit required to be posted with counterparties, if credit-risk-related contingent features were triggered due to credit rating being downgraded below investment grade | $	$ 1,837,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	71,300
2017
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	20,380
2018
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	20,910
2019
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	14,310
2020
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	10,990
2021
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	4,690
2022
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	20