Fair Value of Financial Assets and Liabilities, Changes in Level 3 Nuclear Decommissioning Fund (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period		$ 165,486	$ 120,064	$ 104,600
Purchases		59,597	30,166	55,827
Settlements		(9,611)	(10,080)	(18,622)
Gains (losses) recognized as regulatory assets	[1]	26,806	25,336	10,535
Transfers out of Level 3		0	0	(32,276)	[2]
Balance at end of period		242,278	165,486	120,064
Private equity investments
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period		101,237	62,696	33,250
Purchases		32,029	22,078	24,201
Settlements		0	(286)	0
Gains (losses) recognized as regulatory assets	[1]	24,262	16,749	5,245
Transfers out of Level 3		0	0	0	[2]
Balance at end of period		157,528	101,237	62,696
Real estate
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period		64,249	57,368	39,074
Purchases		27,568	8,088	31,626
Settlements		(9,611)	(9,794)	(18,622)
Gains (losses) recognized as regulatory assets	[1]	2,544	8,587	5,290
Transfers out of Level 3		0	0	0	[2]
Balance at end of period		$ 84,750	64,249	57,368
Asset-backed Securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period			0	2,067
Purchases				0
Settlements				0
Gains (losses) recognized as regulatory assets	[1]			0
Transfers out of Level 3	[2]			(2,067)
Balance at end of period				0
Mortgage-backed securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period			$ 0	30,209
Purchases				0
Settlements				0
Gains (losses) recognized as regulatory assets	[1]			0
Transfers out of Level 3	[2]			(30,209)
Balance at end of period				$ 0

[1] Gains and losses are deferred as a component of the regulatory asset for nuclear decommissioning.

[2] 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 and were subsequently sold during 2013.