Fair Value of Financial Assets and Liabilities, Changes in Level 3 Nuclear Decommissioning Fund (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	$ 146,781	$ 83,730	$ 120,064	$ 104,600
Purchases	12,655	18,078	27,464	33,450
Settlements	(5,876)	0	(5,876)	(9,022)
Gains recognized as regulatory assets	7,417	2,022	19,325	7,078
Transfers out of Level 3	0	0	0	(32,276)	[1]
Balance at end of period	160,977	103,830	160,977	103,830
Private equity investments
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	81,123	45,590	62,696	33,250
Purchases	11,125	6,790	22,078	15,344
Settlements	0	0	0	0
Gains recognized as regulatory assets	4,756	94	12,230	3,880
Transfers out of Level 3	0	0	0	0
Balance at end of period	97,004	52,474	97,004	52,474
Real estate
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	65,658	38,140	57,368	39,074
Purchases	1,530	11,288	5,386	18,106
Settlements	(5,876)	0	(5,876)	(9,022)
Gains recognized as regulatory assets	2,661	1,928	7,095	3,198
Transfers out of Level 3	0	0	0	0
Balance at end of period	63,973	51,356	63,973	51,356
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		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		$ 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.