Inventories and natural gas in storage (Policies)	9 Months Ended
	Sep. 30, 2017
Inventory Disclosure [Abstract]
Inventories and natural gas in storage	Natural gas in storage for the Company's regulated operations is generally carried at lower of cost or net realizable value, or cost using the last-in, first-out method. All other inventories are stated at the lower of cost or net realizable value.