Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	$ 1,003.9	$ 1,024.3
Defined contribution plan, cost recognized	9.5	7.9	$ 7.9
Salary and Wage, Excluding Cost of Good and Service Sold [Abstract]
Allocated share-based compensation expense	4.0	5.9	6.3
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost	9.1	9.6	12.8
Pension Plan [Member] | Directly charged [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost charges to us by Williams	3.7		2.7
Pension - unamortized liability
Defined Benefit Plan Disclosure [Line Items]
Regulatory liability, deferred for future rate treatment	$ 38.9	43.3
Regulatory Liability, Amortization Period	5 years
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other postretirement benefit (income) expense	$ (4.5)	(4.1)	$ (5.9)
Regulatory liabilities	$ 60.9	$ 68.0
Regulatory Liability, Amortization Period	5 years