Fair Value (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Decommissioning funds
Assets (Liabilities):
Balance at the beginning of the period		$ (982)
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)		982
Long-term investments
Assets (Liabilities):
Balance at the beginning of the period		7,713
Total gains or losses (realized/unrealized):
Impairment included in other comprehensive margin (deficit)		887
Liquidations		(8,600)
Interest rate options
Assets (Liabilities):
Balance at the beginning of the period	25,783	69,446
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)	756	(2,586)
Balance at the end of the period	$ 26,539	$ 66,860