Fair Value of Financial Assets and Liabilities, Changes in Level 3 Nuclear Decommissioning Fund (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	$ 74,912		$ 133,191	$ 104,600		$ 130,763
Purchases	9,330		12,920	15,372		31,615
Settlements	(4,723)		(19,148)	(9,022)		(37,643)
Gains (losses) recognized as regulatory assets and liabilities	4,211		2,450	5,056		4,678
Transfers out of Level 3	0	[1]	0	(32,276)	[1]	0
Balance at end of period	83,730		129,413	83,730		129,413
Private equity investments
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	34,506		20,068	33,250		9,203
Purchases	7,298		3,235	8,554		13,390
Settlements	0		0	0		0
Gains (losses) recognized as regulatory assets and liabilities	3,786		0	3,786		710
Transfers out of Level 3	0		0	0	[1]	0
Balance at end of period	45,590		23,303	45,590		23,303
Real Estate
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	40,406		27,905	39,074		26,395
Purchases	2,032		2,271	6,818		3,907
Settlements	(4,723)		0	(9,022)		(1,766)
Gains (losses) recognized as regulatory assets and liabilities	425		2,545	1,270		4,185
Transfers out of Level 3	0		0	0	[1]	0
Balance at end of period	38,140		32,721	38,140		32,721
Asset-backed Securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period			16,547	2,067		16,501
Purchases			0	0		0
Settlements			(9,458)	0		(9,459)
Gains (losses) recognized as regulatory assets and liabilities			(21)	0		26
Transfers out of Level 3			0	(2,067)	[1]	0
Balance at end of period	0		7,068	0		7,068
Mortgage-backed Securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period			68,671	30,209		78,664
Purchases			7,414	0		14,318
Settlements			(9,690)	0		(26,418)
Gains (losses) recognized as regulatory assets and liabilities			(74)	0		(243)
Transfers out of Level 3			0	(30,209)	[1]	0
Balance at end of period	$ 0		$ 66,321	$ 0		$ 66,321

[1] Transfers out of Level 3 into Level 2 were principally due to diminished use of unobservable inputs that were previously significant to these fair value measurements.