Accounting for Derivative Instruments and Hedging Activities Recognized in Statement of Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments, (Loss) Gain [Line Items]
Purchased electricity	$ 114,627	$ 112,543	$ 295,785	$ 287,587
Unrealized (gain) loss on derivative instruments	(17,094)	27,376	(24,934)	(48,316)
Subsidiaries [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Purchased electricity	114,627	112,543	295,785	287,587
Unrealized (gain) loss on derivative instruments	(17,094)	27,376	(24,364)	(45,365)
Not Designated as Hedging Instrument [Member] | Parent Company [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	(12,212)	28,876	(32,390)	(1,143)
Not Designated as Hedging Instrument [Member] | Parent Company [Member] | Other Income (Deductions) [Member] | Interest Expense [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	1,360	(1,232)	2,107	(2,260)
Not Designated as Hedging Instrument [Member] | Parent Company [Member] | Interest Rate Contract [Member] | Interest Expense [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	718	(1,902)	843	676
Not Designated as Hedging Instrument [Member] | Parent Company [Member] | Commodity Contract [Member] | Unrealized (Gain) Loss on Derivative Instruments, Net [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	(17,094)	27,376	(24,934)	(48,316)
Not Designated as Hedging Instrument [Member] | Parent Company [Member] | Commodity Contract [Member] | Electric generation fuel [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	(864)	930	(8,691)	13,568
Not Designated as Hedging Instrument [Member] | Parent Company [Member] | Commodity Contract [Member] | Purchased Electricity [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	3,668	3,704	(1,715)	35,189
Not Designated as Hedging Instrument [Member] | Subsidiaries [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	(14,290)	32,010	(34,770)	3,392
Not Designated as Hedging Instrument [Member] | Subsidiaries [Member] | Commodity Contract [Member] | Unrealized (Gain) Loss on Derivative Instruments, Net [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	(17,094)	27,376	(24,364)	(45,365)
Not Designated as Hedging Instrument [Member] | Subsidiaries [Member] | Commodity Contract [Member] | Electric generation fuel [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	(864)	930	(8,691)	13,568
Not Designated as Hedging Instrument [Member] | Subsidiaries [Member] | Commodity Contract [Member] | Purchased Electricity [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Unrealized (gain) loss on derivative instruments	3,668	3,704	(1,715)	35,189
Cash Flow Hedging [Member] | Parent Company [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	(1,325)	(678)	(2,475)
Cash Flow Hedging [Member] | Parent Company [Member] | Electric generation fuel [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	0	0	0
Cash Flow Hedging [Member] | Subsidiaries [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(122)	(122)	(1,348)	(3,030)
Cash Flow Hedging [Member] | Subsidiaries [Member] | Electric generation fuel [Member]
Derivative Instruments, (Loss) Gain [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0	$ 0	$ 0	$ 0