Restructuring Expected Costs, Cost Recognized, and Remaining Costs to be Recognized - 2014 Projects (Details) - USD ($) $ in Thousands	3 Months Ended							6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2015	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restructuring Cost and Reserve [Line Items]
Restructuring Charges	$ 587	$ (4,234)	$ (5,181)		$ 57	$ (8,657)	$ 6,383	$ (9,415)	$ (2,274)	$ (8,828)	$ (2,217)	$ (7,263)	$ 49,482	$ 36,347
Employee severance costs
Restructuring Cost and Reserve [Line Items]
Restructuring Charges												(12,151)
Asset Impairment and Net (Gain) Loss on Sale or Disposal [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Charges												976
Other restructuring costs
Restructuring Cost and Reserve [Line Items]
Restructuring Charges												3,912
2014 Project [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												50,236
Restructuring Charges				$ 54,400								(7,263)	54,445
Restructuring and Related Cost, Expected Cost Remaining												3,054
2014 Project [Member] | Electricity Operating Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												24,615
Restructuring Charges												(7,253)	28,996
Restructuring and Related Cost, Expected Cost Remaining												2,872
2014 Project [Member] | Gas Operating Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												11,936
Restructuring Charges												(287)	12,142
Restructuring and Related Cost, Expected Cost Remaining												81
2014 Project [Member] | Water Operating Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												1,953
Restructuring Charges												778	1,162
Restructuring and Related Cost, Expected Cost Remaining												13
2014 Project [Member] | Corporate, Non-Segment [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												11,732
Restructuring Charges												(501)	12,145
Restructuring and Related Cost, Expected Cost Remaining												88
2014 Project [Member] | Employee severance costs
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												34,373
Restructuring Charges												(12,151)	46,524
Restructuring and Related Cost, Expected Cost Remaining												0
2014 Project [Member] | Asset Impairment and Net (Gain) Loss on Sale or Disposal [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												8,880
Restructuring Charges												976	7,904
Restructuring and Related Cost, Expected Cost Remaining												0
2014 Project [Member] | Other restructuring costs
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Expected Cost												6,983
Restructuring Charges												3,912	$ 17
Restructuring and Related Cost, Expected Cost Remaining												$ 3,054