Accounting for Derivative Instruments and Hedging Activities Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 15,187	$ 996
Unrealized gain on derivative instruments	52,910		$ 46,507
Assets, Long-term	4,725		2,512
Derivative Liability, Current	32,452		46,661
Unrealized loss on derivative instruments	7,184		11,095
Subsidiaries [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	15,187	996
Unrealized gain on derivative instruments	52,910		46,507
Assets, Long-term	4,725		2,512
Derivative Liability, Current	32,452		46,661
Unrealized loss on derivative instruments	7,184		11,095
Commodity Contract [Member]
Derivative [Line Items]
Assets	57,635		49,019
Derivative Liability	39,636		57,756
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	64,807	(9,069)
Unrealized gain on derivative instruments	52,910		46,507
Assets, Long-term	4,725		2,512
Assets	57,635		49,019
Derivative Liability, Current	32,452		46,661
Derivative Liability	39,636		57,756
Unrealized loss on derivative instruments	7,184		11,095
Not Designated as Hedging Instrument [Member] \| Electric Portfolio [Member]
Derivative [Line Items]
Assets	37,728		33,287
Derivative Liability	16,538		27,284
Not Designated as Hedging Instrument [Member] \| Natural Gas Portfolio [Member]
Derivative [Line Items]
Assets	19,907		15,732
Derivative Liability	23,098		$ 30,472
Not Designated as Hedging Instrument [Member] \| Unrealized (Gain) Loss on Derivative Instruments, Net [Member] \| Electricity, US Regulated [Member]
Derivative [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 226	$ (1,316)

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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