Regulation and Rates (Details) - Subsidiaries [Member] $ in Millions															9 Months Ended		12 Months Ended
	Jan. 01, 2018 USD ($)	Nov. 01, 2017 USD ($)	Oct. 01, 2017 USD ($)	Sep. 15, 2017 USD ($)	May 01, 2017 USD ($)	Apr. 03, 2017 USD ($)	Jan. 13, 2017 USD ($)	Jan. 01, 2017 USD ($)	Dec. 01, 2016 USD ($)	Nov. 01, 2016 USD ($)	May 01, 2016 USD ($)	Jan. 01, 2016 USD ($)	Oct. 01, 2015 USD ($)	Jun. 25, 2013	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2013
Regulatory Assets [Line Items]
Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities														7.77%
Public Utilities, Rate Case, Approved Effective Common Equity in Capital Structure														48.00%
Public Utilities, Rate Case, Approved Effective Return on Equity														9.80%
Electric [Member]
Regulatory Assets [Line Items]
Storm Damage Costs Incurred During Period															$ 21.1	$ 15.6
Storm Damage Costs Deferred During Period															12.4	6.5
Public Utilities, Rate Case, Deferred Storm Costs Threshold															8.0
Residential Exchange Benefit [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage													2.40%
Public Utilities, Approved credit to be passed back to eligible customers													$ (76.4)
Federal Incentive Tracker Tariff [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage								0.30%				(0.20%)
Public Utilities, Approved credit to be passed back to eligible customers								$ (51.7)				$ (57.3)
Power Cost Only Rate Case (PCORC) [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage									(1.70%)
Public Utilities, Approved Rate Increase (Decrease), Amount									$ (37.3)
Energy Conservation Costs [Member] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage					(0.10%)						0.30%
Public Utilities, Approved Rate Increase (Decrease), Amount					$ (1.0)						$ 2.9
Energy Conservation Costs [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage					0.70%						(0.50%)
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 16.5						$ (11.7)
Property tax tracker [Member] [Domain] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage					(0.10%)						0.40%
Public Utilities, Approved Rate Increase (Decrease), Amount					$ (1.1)						$ 3.5
Property tax tracker [Member] [Domain] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage					(0.04%)						0.30%
Public Utilities, Approved Rate Increase (Decrease), Amount					$ (0.9)						$ 5.7
Cost recovery mechanism [Member] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage										0.60%
Public Utilities, Approved Rate Increase (Decrease), Amount										$ 5.6
Purchased Gas Adjustment (PGA) [Member] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage										(0.40%)
Public Utilities, Approved Rate Increase (Decrease), Amount										$ (4.1)
Decoupling Mechanism [Member]
Regulatory Assets [Line Items]
Public Utilities, Percentage of Earning to Be Shared Between the Company and Customers														50.00%
Decoupling Mechanism [Member] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage					2.40%						2.80%	2.20%
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 22.4						$ 25.4						$ 2.2	$ 5.5
Decoupling Mechanism [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage					2.00%						1.00%	3.00%
Public Utilities, Approved Rate Increase (Decrease), Amount					$ 41.9						$ 20.8						11.9	11.9
Decoupling Mechanism [Member] | Maximum [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage												3.00%
Decoupling Mechanism [Member] | Deferred Revenue [Domain] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount																	$ 47.4	$ 28.7
General Rate Case [Member]
Regulatory Assets [Line Items]
Regulated Utility, Allowed Rate of Return on Net Regulatory Assets and Liabilities				7.60%			7.74%
Regulated Utility, After-tax Allowed Rate of Return on Net Regulatory Assets and Liabilities				6.55%			6.69%
Public Utilities, Requested Equity Capital Structure, Percentage				48.50%			48.50%
Public Utilities, Requested Return on Equity, Percentage				9.50%			9.80%
Number of new mechanisms							2
Number of parties that agreed to settle				10
Number of parties total				11
Number of contested issues not settled				4
General Rate Case [Member] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 22.3
Public Utilities, Requested Rate Increase (Decrease), Percentage							2.40%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount				$ 35.5		$ 29.3
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage				3.80%		3.20%
General Rate Case [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 86.3
Public Utilities, Requested Rate Increase (Decrease), Percentage							4.10%
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount				$ 20.2		$ 67.9
Public Utilities, Requested Rate Increase (Decrease), Amended, Percentage				0.90%		3.20%
Customers share [Member] | Range 2 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			50.00%
Customers share [Member] | Range 3 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			90.00%
Customers share [Member] | Range 4 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			95.00%
Customers share [Member] | Range 1 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			0.00%
Company's share [Member] | Range 2 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			50.00%
Company's share [Member] | Range 3 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			10.00%
Company's share [Member] | Range 4 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			5.00%
Company's share [Member] | Range 1 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage																			100.00%
Under Recovery [Member] | Customers share [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount																0.0
Under Recovery [Member] | Customers share [Member] | Range 2 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								50.00%
Under Recovery [Member] | Customers share [Member] | Range 3 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								90.00%
Under Recovery [Member] | Customers share [Member] | Range 1 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								0.00%
Under Recovery [Member] | Company's share [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount															(8.9)	$ (1.4)
Under Recovery [Member] | Company's share [Member] | Range 2 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								50.00%
Under Recovery [Member] | Company's share [Member] | Range 3 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								10.00%
Under Recovery [Member] | Company's share [Member] | Range 1 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								100.00%
Over Recovery [Member] | Customers share [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount															$ 0.0
Over Recovery [Member] | Customers share [Member] | Range 2 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								65.00%
Over Recovery [Member] | Customers share [Member] | Range 3 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								90.00%
Over Recovery [Member] | Customers share [Member] | Range 1 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								0.00%
Over Recovery [Member] | Company's share [Member] | Range 2 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								35.00%
Over Recovery [Member] | Company's share [Member] | Range 3 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								10.00%
Over Recovery [Member] | Company's share [Member] | Range 1 [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Percentage								100.00%
Deferral Trigger [Member] [Member] | Minimum [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount								$ 30.0
Deferral Trigger [Member] [Member] | Maximum [Member] | Electric [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount								$ 20.0
Scenario, Forecast [Member] | Federal Incentive Tracker Tariff [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Percentage	0.20%
Public Utilities, Requested credit to be passed back to eligible customers	$ (48.2)
Subsequent Event [Member] | Residential Exchange Benefit [Member] | Electric [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage			(0.60%)
Public Utilities, Approved credit to be passed back to eligible customers			$ (80.8)
Subsequent Event [Member] | Cost recovery mechanism [Member] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage		0.50%
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 4.9
Subsequent Event [Member] | Purchased Gas Adjustment (PGA) [Member] | Natural Gas [Member]
Regulatory Assets [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage		(3.30%)
Public Utilities, Approved Rate Increase (Decrease), Amount		$ (30.8)