Property Plant and Equipment Property Plant and Equipment (Tables)

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Public Utility Property, Plant, and Equipment

(Millions of Dollars)	Dec. 31, 2018			Dec. 31, 2017
Property, plant and equipment
Electric plant	$	13,604.5		$	12,627.6
Natural gas plant	4,387.6			4,102.1
Common and other property	1,023.7			1,022.3
Plant to be retired (a)	321.9			11.0
CWIP	573.3			1,014.3
Total property, plant and equipment	19,911.0			18,777.3
Less accumulated depreciation	(4,791.0		)	(4,751.5		)
	$	15,120.0		$	14,025.8

Schedule of Jointly Owned Utility Plants

(Millions of Dollars)	Plant in Service		Accumulated Depreciation		CWIP		Percent Owned
Electric Generation:
Hayden Unit 1	$	152.8	$	76.5	$	—	76	%
Hayden Unit 2	148.9		68.0		—		37
Hayden Common Facilities	40.8		20.9		—		53
Craig Units 1 and 2	81.0		40.0		—		10
Craig Common Facilities	39.1		20.9		—		7
Comanche Unit 3	886.3		130.7		—		67
Comanche Common Facilities	27.9		2.5		0.1		82
Electric Transmission:
Transmission and other facilities	182.8		63.2		0.7		Various
Gas Transportation:
Rifle, CO to Avon, CO	21.5		7.2		0.1		60
Gas Tran Compressor	8.4		0.9		—		50
Total	$	1,589.5	$	430.8	$	0.9