Fair Value Measurement (Details 2) (USD $)			1 Months Ended	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Feb. 29, 2012 Auction rate securities	Sep. 30, 2011 Decommissioning funds	Sep. 30, 2012 Decommissioning funds	Sep. 30, 2011 Decommissioning funds	Sep. 30, 2011 Long-term investments	Sep. 30, 2012 Long-term investments	Sep. 30, 2011 Long-term investments	Sep. 30, 2012 Interest rate options	Sep. 30, 2012 Interest rate options
Assets (Liabilities):
Balance at the beginning of the period				$ (505,000)	$ (982,000)	$ (452,000)	$ 8,048,000	$ 7,713,000	$ 8,671,000	$ 39,215,000	$ 69,446,000
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)				(527,000)	982,000	(580,000)			127,000	(9,294,000)	(39,525,000)
Impairment included in other comprehensive margin (deficit)							50,000	887,000
Liquidations							(400,000)	(8,600,000)	(1,100,000)
Balance at the end of the period				(1,032,000)		(1,032,000)	7,698,000		7,698,000	29,921,000	29,921,000
Investments sold	325,502,000	325,127,000	8,600,000
Loss on sale of investments			$ 1,075,000
Period during which loss on investments will be charged to income			4 years