Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	$ 1,024.3	$ 1,032.0
Defined contribution plan, cost recognized	7.9	7.9	$ 7.7
Salary and Wage, Excluding Cost of Good and Service Sold [Abstract]
Allocated share-based compensation expense	5.9	6.3	5.7
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost	9.6	12.8	15.6
Pension Plan [Member] | Directly charged [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost charges to us by Williams		2.7	7.6
Pension Plan [Member] | Allocated corporate expenses [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost charges to us by Williams		2.7	4.6
Pension - unamortized liability
Defined Benefit Plan Disclosure [Line Items]
Regulatory liability, deferred for future rate treatment	43.3	48.5
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Other postretirement benefit (income) expense	(4.1)	(5.9)	$ (10.9)
Regulatory liabilities	$ 68.0	$ 79.8
Regulatory Liability, Amortization Period	5 years 4 months 13 days