Accounting Policies New Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gas stored underground	$ 81.7		$ 81.7		$ 86.3
Other assets	138.5		138.5		139.8
Other liabilities	76.0		76.0		65.0
Partners' Capital	4,831.0	$ 4,668.2	4,831.0	$ 4,668.2	$ 4,724.8	$ 4,530.9
Transportation revenues	245.8	266.4	792.8	852.0
Storage, parking and lending	21.1	23.2	67.8	79.8
Other	11.0	10.9	38.0	53.3
Total operating revenues	277.9	300.5	898.6	985.1
Fuel and transportation expense	3.5	9.0	12.0	42.7
Operating income	80.8	110.0	307.2	341.0
Net income	37.9	69.8	177.3	212.8
Miscellaneous other income, net	0.7	1.1	1.3	3.5
Administrative and general	32.8	27.1	100.6	96.9
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets (gas stored underground)	0.4		0.4
Gas stored underground	81.7		81.7
Other assets	138.5		138.5
Other liabilities	76.0		76.0
Partners' Capital	4,831.0		4,831.0
Transportation revenues	245.8		792.8
Storage, parking and lending	21.1		67.8
Other	11.0		38.0
Total operating revenues	277.9		898.6
Fuel and transportation expense	3.5		12.0
Operating income	80.8		307.2
Net income	37.9		177.3
Accounting Standards Update 2014-09 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets (gas stored underground)	4.0		4.0
Gas stored underground	0.3		0.3
Other assets	(0.1)		(0.1)
Other liabilities	(8.1)		(8.1)
Partners' Capital	12.2		12.2
Transportation revenues	5.5	(6.5)	17.5	(18.2)
Storage, parking and lending	0.1		0.3
Other	0.1	6.5	(4.5)	18.2
Total operating revenues	5.7		13.3
Fuel and transportation expense	4.6		13.8
Operating income	1.0		(0.6)
Net income	1.0		(0.6)
Accounting Standards Update 2014-09 | Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets (gas stored underground)	4.4		4.4
Gas stored underground	82.0		82.0
Other assets	138.4		138.4
Other liabilities	67.9		67.9
Partners' Capital	4,843.2		4,843.2
Transportation revenues	251.3		810.3
Storage, parking and lending	21.2		68.1
Other	11.1		33.5
Total operating revenues	283.6		911.9
Fuel and transportation expense	8.1		25.8
Operating income	81.8		306.6
Net income	$ 38.9		$ 176.7
Accounting Standards Update 2017-07 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Miscellaneous other income, net		0.8		1.8
Administrative and general		$ 0.8		$ 1.8