Accumulated Other Comprehensive Income - Reclassification Location (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended			3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended				6 Months Ended		3 Months Ended				6 Months Ended				3 Months Ended				6 Months Ended		3 Months Ended				6 Months Ended				3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Subsidiaries [Member]	Jun. 30, 2013 Subsidiaries [Member]	Jun. 30, 2014 Subsidiaries [Member]	Jun. 30, 2013 Subsidiaries [Member]	Dec. 31, 2013 Subsidiaries [Member]	Jun. 30, 2014 Accumulated Defined Benefit Plans Adjustment [Member]	Jun. 30, 2014 Accumulated Defined Benefit Plans Adjustment [Member]	Mar. 31, 2014 Accumulated Defined Benefit Plans Adjustment [Member]	Dec. 31, 2013 Accumulated Defined Benefit Plans Adjustment [Member]	Jun. 30, 2014 Accumulated Defined Benefit Plans Adjustment [Member] Subsidiaries [Member]	Jun. 30, 2014 Accumulated Defined Benefit Plans Adjustment [Member] Subsidiaries [Member]	Mar. 31, 2014 Accumulated Defined Benefit Plans Adjustment [Member] Subsidiaries [Member]	Dec. 31, 2013 Accumulated Defined Benefit Plans Adjustment [Member] Subsidiaries [Member]	Jun. 30, 2014 Reclassification out of Accumulated Other Comprehensive Income [Member]	Jun. 30, 2013 Reclassification out of Accumulated Other Comprehensive Income [Member]	Jun. 30, 2014 Reclassification out of Accumulated Other Comprehensive Income [Member]	Jun. 30, 2013 Reclassification out of Accumulated Other Comprehensive Income [Member]	Jun. 30, 2014 Reclassification out of Accumulated Other Comprehensive Income [Member] Subsidiaries [Member]	Jun. 30, 2013 Reclassification out of Accumulated Other Comprehensive Income [Member] Subsidiaries [Member]	Jun. 30, 2014 Reclassification out of Accumulated Other Comprehensive Income [Member] Subsidiaries [Member]	Jun. 30, 2013 Reclassification out of Accumulated Other Comprehensive Income [Member] Subsidiaries [Member]	Jun. 30, 2014 Interest Rate Contract [Member] Interest Expense [Member]	Jun. 30, 2013 Interest Rate Contract [Member] Interest Expense [Member]	Jun. 30, 2014 Interest Rate Contract [Member] Interest Expense [Member]	Jun. 30, 2013 Interest Rate Contract [Member] Interest Expense [Member]	Jun. 30, 2014 Interest Rate Contract [Member] Interest Expense [Member] Subsidiaries [Member]	Jun. 30, 2013 Interest Rate Contract [Member] Interest Expense [Member] Subsidiaries [Member]	Jun. 30, 2014 Interest Rate Contract [Member] Interest Expense [Member] Subsidiaries [Member]	Jun. 30, 2013 Interest Rate Contract [Member] Interest Expense [Member] Subsidiaries [Member]	Jun. 30, 2014 Electric Derivative [Member] Purchased Electricity [Member]	Jun. 30, 2013 Electric Derivative [Member] Purchased Electricity [Member]	Jun. 30, 2014 Electric Derivative [Member] Purchased Electricity [Member]	Jun. 30, 2013 Electric Derivative [Member] Purchased Electricity [Member]	Jun. 30, 2014 Electric Derivative [Member] Purchased Electricity [Member] Subsidiaries [Member]	Jun. 30, 2013 Electric Derivative [Member] Purchased Electricity [Member] Subsidiaries [Member]	Jun. 30, 2014 Electric Derivative [Member] Purchased Electricity [Member] Subsidiaries [Member]	Jun. 30, 2013 Electric Derivative [Member] Purchased Electricity [Member] Subsidiaries [Member]	Jun. 30, 2014 Qualified Pension Benefits [Member] Pension Cost [Member]	Jun. 30, 2013 Qualified Pension Benefits [Member] Pension Cost [Member]	Jun. 30, 2014 Qualified Pension Benefits [Member] Pension Cost [Member]	Jun. 30, 2013 Qualified Pension Benefits [Member] Pension Cost [Member]	Jun. 30, 2014 Qualified Pension Benefits [Member] Pension Cost [Member] Subsidiaries [Member]	Jun. 30, 2013 Qualified Pension Benefits [Member] Pension Cost [Member] Subsidiaries [Member]	Jun. 30, 2014 Qualified Pension Benefits [Member] Pension Cost [Member] Subsidiaries [Member]	Jun. 30, 2013 Qualified Pension Benefits [Member] Pension Cost [Member] Subsidiaries [Member]	Jun. 30, 2014 Amortization of Net Gain (Loss) [Member] Pension Cost [Member]		Jun. 30, 2013 Amortization of Net Gain (Loss) [Member] Pension Cost [Member]		Jun. 30, 2014 Amortization of Net Gain (Loss) [Member] Pension Cost [Member]	Jun. 30, 2013 Amortization of Net Gain (Loss) [Member] Pension Cost [Member]	Jun. 30, 2014 Amortization of Net Gain (Loss) [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2013 Amortization of Net Gain (Loss) [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2014 Amortization of Net Gain (Loss) [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2013 Amortization of Net Gain (Loss) [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2014 Amortization of Prior Service Cost [Member] Pension Cost [Member]		Jun. 30, 2013 Amortization of Prior Service Cost [Member] Pension Cost [Member]		Jun. 30, 2014 Amortization of Prior Service Cost [Member] Pension Cost [Member]	Jun. 30, 2013 Amortization of Prior Service Cost [Member] Pension Cost [Member]	Jun. 30, 2014 Amortization of Prior Service Cost [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2013 Amortization of Prior Service Cost [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2014 Amortization of Prior Service Cost [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2013 Amortization of Prior Service Cost [Member] Pension Cost [Member] Subsidiaries [Member]		Jun. 30, 2014 Derivative [Member]	Jun. 30, 2014 Derivative [Member]	Mar. 31, 2014 Derivative [Member]	Dec. 31, 2013 Derivative [Member]	Jun. 30, 2014 Derivative [Member] Subsidiaries [Member]	Jun. 30, 2014 Derivative [Member] Subsidiaries [Member]	Mar. 31, 2014 Derivative [Member] Subsidiaries [Member]	Dec. 31, 2013 Derivative [Member] Subsidiaries [Member]	Jun. 30, 2014 Interest Rate Swap [Member]	Jun. 30, 2014 Interest Rate Swap [Member]	Mar. 31, 2014 Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap [Member]	Jun. 30, 2014 Interest Rate Swap [Member] Subsidiaries [Member]	Jun. 30, 2014 Interest Rate Swap [Member] Subsidiaries [Member]	Mar. 31, 2014 Interest Rate Swap [Member] Subsidiaries [Member]	Dec. 31, 2013 Interest Rate Swap [Member] Subsidiaries [Member]	Jun. 30, 2014 Comprehensive Income [Member]	Jun. 30, 2014 Comprehensive Income [Member]	Mar. 31, 2014 Comprehensive Income [Member]	Dec. 31, 2013 Comprehensive Income [Member]	Jun. 30, 2014 Comprehensive Income [Member] Subsidiaries [Member]	Jun. 30, 2014 Comprehensive Income [Member] Subsidiaries [Member]	Mar. 31, 2014 Comprehensive Income [Member] Subsidiaries [Member]	Dec. 31, 2013 Comprehensive Income [Member] Subsidiaries [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ 47,164		$ 47,164		$ 47,715	$ (91,438)		$ (91,438)		$ (95,739)	$ 47,522	$ 47,522	$ 48,305	$ 48,514	$ (83,980)	$ (83,980)	$ (85,404)	$ (87,405)																																																													$ (358)	$ (358)	$ (358)	$ (705)	$ (1,309)	$ (1,309)	$ (1,309)	$ (2,027)	$ 0	$ 0	$ 0	$ (94)	$ (6,149)	$ (6,149)	$ (6,228)	$ (6,307)	$ 47,164	$ 47,164	$ 47,947	$ 47,715	$ (91,438)	$ (91,438)	$ (92,941)	$ (95,739)
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent																											0	(1,325)	(144)	(2,639)	(122)	(122)	(244)	(244)	0	0	(534)	164	0	0	(1,104)	(2,786)	386	(678)	709	(1,210)	(3,135)	(5,272)	(6,213)	(10,479)	(98)	[1]	(1,177)	[1]	(456)	(2,209)	(3,516)	[2]	(5,661)	[2]	(6,975)	[2]	(11,260)	[2]	484	[1]	499	[1]	1,165	999	381	[2]	389	[2]	762	[2]	781	[2]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent																											0	464	50	924	43	43	86	86	0	0	187	(57)	0	0	386	975	(135)	237	(248)	422	1,097	1,845	2,175	3,666
Comprehensive income (loss)	40,330	659	148,154	169,228		59,337	28,621	183,217	213,835										251	(1,302)	20	(2,396)	(2,117)	(3,506)	(4,914)	(8,782)	0	(861)	(94)	(1,715)	(79)	(79)	(158)	(158)	0	0	(347)	107	0	0	(718)	(1,811)	251	(441)	461	(788)	(2,038)	(3,427)	(4,038)	(6,813)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax											(615)	(615)			(615)	(615)																																																															0	0			0	0			0	0			0	0			(615)	(615)			(615)	(615)
Other comprehensive income (loss) before reclassifications											(168)	(377)			2,039	4,040																																																															0	347			0	718			0	94			79	158			(168)	64			2,118	4,916
Amortization Of Financing Cash Flow Hedge Contracts To Earnings During Period Net Of Tax						79	79	158	158
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0	(347)	107		0	0	(718)	(1,811)
Other Comprehensive Income Reclassification Of Net Unrealized Gain Loss On Interest Rate Swaps During Period Net Of Tax	0	(861)	(94)	(1,715)
Amounts reclassified from accumulated other comprehensive income (loss), net of tax											$ (783)	$ (992)			$ 1,424	$ 3,425																																																															$ 0	$ 347			$ 0	$ 718			$ 0	$ 94			$ 79	$ 158			$ (783)	$ (551)			$ 1,503	$ 4,301

[1] (a) These accumulated OCI components are included in the computation of net periodic pension cost (see Note 5 for additional details).

[2] (a) These accumulated OCI components are included in the computation of net periodic pension cost (see Note 5 for additional details).