Basis of Presentation of Interim Financial Statements (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Cost of Sales [Abstract]
Gas purchases		$ 2.0	$ 0.3	$ 51.1	$ 43.5
Operator service fee	[1]	77.7	80.7	236.4	241.0
Transportation and storage	[1]	17.8	18.1	57.5	58.4
Gathering		6.0	5.7	17.4	16.7
Royalties		5.7	7.6	16.0	26.4
Storage (injection), net		(44.2)	(42.8)	(23.4)	(28.6)
Purchased-gas account adjustment		(26.6)	(26.4)	5.5	0.9
Other		1.3	1.2	3.9	3.7
Total cost of natural gas sold (excluding operating expenses shown separately)		$ 39.7	$ 44.4	$ 364.4	$ 362.0

[1] See Note 9 for amounts attributable to affiliates