Derivative Instruments - Fair value of derivative instruments not designated as hedging (Details) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Interest rate options
Assets:
Fair value of assets	$ 500	$ 1,010,000
Natural gas swaps
Liabilities:
Fair Value of liabilities	2,823,000	22,848,000
Not designated as hedge | Interest rate options | Other deferred charges
Assets:
Fair value of assets		1,010,000
Not designated as hedge | Natural gas swaps | Other deferred charges
Assets:
Fair value of assets	1,142,000
Not designated as hedge | Natural gas swaps | Other current assets
Assets:
Fair value of assets	633,000
Not designated as hedge | Natural gas swaps | Other current liabilities
Liabilities:
Fair Value of liabilities	909,000	$ 22,848,000
Not designated as hedge | Natural gas swaps | Other deferred credits
Liabilities:
Fair Value of liabilities	$ 3,689,000