Fair Value: (Details 2) (USD $)			1 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Feb. 29, 2012 Loss on auction rate securities	Feb. 29, 2012 Auction rate securities	Feb. 15, 2012 Auction rate securities	Dec. 31, 2012 Nuclear decommissioning trust funds	Dec. 31, 2011 Nuclear decommissioning trust funds	Dec. 31, 2012 Long-term investments	Dec. 31, 2011 Long-term investments	Dec. 31, 2012 Interest rate options	Dec. 31, 2011 Interest rate options
Assets (Liabilities):
Balance at the beginning of the period						$ (982,000)	$ (452,000)	$ 7,713,000	$ 8,671,000	$ 69,446,000
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)						982,000	(530,000)	887,000	142,000	(43,663,000)	(30,554,000)
Purchases, issuances, liquidations								(8,600,000)	(1,100,000)		100,000,000
Balance at the end of the period							(982,000)		7,713,000	25,783,000	69,446,000
Investments sold	330,627,000	325,127,000			8,600,000
Loss on sale of investments				$ 1,075,000
Period over which loss is charged to income			P4Y