Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	$ 1,007.1	$ 458.5
Defined contribution plan, cost recognized	7.7	6.5	$ 6.6
Salaries, Wages and Officers' Compensation [Abstract]
Allocated share-based compensation expense	5.7	4.0	4.0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension cost	15.6	8.7	13.5
Pension Plan [Member] | Directly charged [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension costs charges to us by Williams	7.6
Pension Plan [Member] | Allocated corporate expenses [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension costs charges to us by Williams	4.6
Pension Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liability, deferred for future rate treatment	32.5	21.3
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liability, deferred for future rate treatment	65.4	52.0
Other postretirement benefit (income) expense	(10.9)	(12.0)	$ (11.9)
Regulatory liabilities	73.9	63.0
Regulatory liability, being amortized	$ 8.5	$ 11.0
Regulatory Liability, Amortization Period	8 years