Accumulated Other Comprehensive Income - Reclassification Location (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Comprehensive income (loss)	$ 107,824		$ 168,569
Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Comprehensive income (loss)	123,881		185,214
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Comprehensive income (loss)	(232)		(1,094)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Comprehensive income (loss)	(2,798)		(5,276)
Interest Rate Contract [Member] | Interest Expense [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(144)		(1,314)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	50		460
Comprehensive income (loss)	(94)		(854)
Interest Rate Contract [Member] | Interest Expense [Member] | Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(122)		(122)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	43		43
Comprehensive income (loss)	(79)		(79)
Electric Derivative [Member] | Purchased Electricity [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(534)		164
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	187		(57)
Comprehensive income (loss)	(347)		107
Electric Derivative [Member] | Purchased Electricity [Member] | Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(1,104)		(2,786)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	386		975
Comprehensive income (loss)	(718)		(1,811)
Qualified Pension Benefits [Member] | Pension Cost [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	322		(533)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(113)		186
Comprehensive income (loss)	209		(347)
Qualified Pension Benefits [Member] | Pension Cost [Member] | Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(3,080)		(5,210)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	1,079		1,824
Comprehensive income (loss)	(2,001)		(3,386)
Amortization of Net Gain (Loss) [Member] | Pension Cost [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(162)	[1]	(1,032)	[1]
Amortization of Net Gain (Loss) [Member] | Pension Cost [Member] | Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(3,461)	[2]	(5,599)	[2]
Amortization of Prior Service Cost [Member] | Pension Cost [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	484	[1]	499	[1]
Amortization of Prior Service Cost [Member] | Pension Cost [Member] | Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	$ 381	[2]	$ 389	[2]

[1] (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see Note 4 for additional details).

[2] (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see Note 4 for additional details).