CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Other comprehensive income (loss):
Net tax on reclassification of net unrealized loss on interest rate swaps during the period, tax		$ 3,817		$ 5,167
Income tax (benefit) expense	(957)	20,000	69,634	52,346
Net unrealized gain (loss) from pension and postretirement plans, tax	(596)	(56)	(409)	(112)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	0	122	(107)	(73)
Reclassification of net unrealized loss on energy derivative instruments during the period, tax		66		(39)
Purchased electricity	112,543	156,098	287,587	355,213
Subsidiaries [Member]
Other comprehensive income (loss):
Income tax (benefit) expense	11,180	31,087	90,941	77,301
Net unrealized gain (loss) from pension and postretirement plans, tax	1,011	661	2,835	1,962
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	0	2,869	1,811	4,791
Reclassification of net unrealized loss on energy derivative instruments during the period, tax		1,545		2,580
Purchased electricity	112,543	156,098	287,587	355,213
Amortization Of Financing Cash Flow Hedge Contracts To Earnings Tax		$ 43	$ 85	$ 85