Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	$ 1,032.0	$ 1,007.1
Defined contribution plan, cost recognized	7.9	7.7	$ 6.5
Salary and Wage, Excluding Cost of Good and Service Sold [Abstract]
Allocated share-based compensation expense	6.3	5.7	4.0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost	12.8	15.6	8.7
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 Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liability, deferred for future rate treatment	48.5	32.5
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liability, deferred for future rate treatment	73.9	65.4
Other postretirement benefit (income) expense	(5.9)	(10.9)	$ (12.0)
Regulatory liabilities	79.8	73.9
Regulatory liability, being amortized	$ 5.9	$ 8.5
Regulatory Liability, Amortization Period	8 years