Accounting for Derivative Instruments and Hedging Activities Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Unrealized gain on derivative instruments	$ 16,605	$ 54,341
Derivative Liability, Current	49,820	44,310
Assets, Long-term	2,877	8,738
Unrealized loss on derivative instruments	15,578	16,261
Subsidiaries [Member]
Derivative [Line Items]
Unrealized gain on derivative instruments	16,605	54,341
Derivative Liability, Current	49,820	44,170
Assets, Long-term	2,877	8,738
Unrealized loss on derivative instruments	15,578	16,261
Commodity Contract [Member]
Derivative [Line Items]
Assets	19,482	63,079
Derivative Liability	65,398	60,430
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Assets	19,482	63,079
Derivative Liability	65,398	60,571
Not Designated as Hedging Instrument [Member] | Parent Company [Member]
Derivative [Line Items]
Unrealized gain on derivative instruments	16,605	54,341
Derivative Liability, Current	49,820	44,310
Assets, Long-term	2,877	8,738
Unrealized loss on derivative instruments	15,578	16,261
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	450,000
Assets	0	0
Derivative Liability	0	141
Not Designated as Hedging Instrument [Member] | Electric Portfolio [Member]
Derivative [Line Items]
Assets	11,656	36,460
Derivative Liability	39,622	41,329
Not Designated as Hedging Instrument [Member] | Natural Gas Portfolio [Member]
Derivative [Line Items]
Assets	7,826	26,619
Derivative Liability	$ 25,776	$ 19,101