Fair Value (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 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	26,539	66,860	25,783	69,446
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)	17,141	(27,645)	17,897	(30,231)
Balance at the end of the period	$ 43,680	$ 39,215	$ 43,680	$ 39,215