Fair Value of Financial Assets and Liabilities, Changes in Level 3 Nuclear Decommissioning Fund (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	$ 120,064	$ 104,600
Purchases	12,429	6,042
Settlements	0	(4,299)
Gains recognized as regulatory assets	4,262	845
Transfers out of Level 3	0	(32,276)	[1]
Balance at end of period	136,755	74,912
Private equity investments
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	62,696	33,250
Purchases	8,769	1,256
Settlements	0	0
Gains recognized as regulatory assets	2,336	0
Transfers out of Level 3	0	0
Balance at end of period	73,801	34,506
Real estate
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	57,368	39,074
Purchases	3,660	4,786
Settlements	0	(4,299)
Gains recognized as regulatory assets	1,926	845
Transfers out of Level 3	0	0
Balance at end of period	62,954	40,406
Asset-backed Securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period		2,067
Purchases		0
Settlements		0
Gains recognized as regulatory assets		0
Transfers out of Level 3		(2,067)	[1]
Balance at end of period		0
Mortgage-backed Securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period		30,209
Purchases		0
Settlements		0
Gains recognized as regulatory assets		0
Transfers out of Level 3		(30,209)	[1]
Balance at end of period		$ 0

[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.