Accounting for Derivative Instruments and Hedging Activities Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Unrealized gain on derivative instruments	$ 23,718	$ 22,247
Assets, Long-term	3,443	2,158
Derivative Liability, Current	67,513	64,859
Unrealized loss on derivative instruments	16,567	21,235
Subsidiaries [Member]
Derivative [Line Items]
Unrealized gain on derivative instruments	23,718	22,247
Assets, Long-term	3,443	2,158
Derivative Liability, Current	67,513	64,859
Unrealized loss on derivative instruments	16,567	21,235
Commodity Contract [Member]
Derivative [Line Items]
Assets	27,161	24,405
Derivative Liability	$ 84,080	$ 86,094