Net Earnings per Common Unit (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings per common unit:
Net earnings	$ 49.4	$ 22.8	$ 39.5	$ 42.2	$ 44.1	$ 28.8	$ 98.7	$ 37.7	$ 153.9	$ 209.3	$ 306.9
(Distributions in excess of earnings) undistributed earnings									$ (0.7)	$ 13.4	$ (2.0)
Weighted-average common units outstanding									18,500,000	18,500,000	18,500,000
Net earnings per common unit	$ 1.44	$ 0.92	$ 1.48	$ 2.10	$ 1.85	$ 1.04	$ 3.22	$ 1.44	$ 5.93	$ 7.56	$ 10.06
Dilutive TNCLP units outstanding									0	0	0
Common Units | Common units
Basic earnings per common unit:
Distributions declared									$ 110.3	$ 130.9	$ 187.4
Allocation percentage for undistributed earnings or distributions in excess of earnings									71.33%	66.82%	60.67%
Allocation of (distributions in excess of earnings) undistributed earnings									$ (0.5)	$ 9.0	$ (1.2)
Net earnings allocated									109.8	139.9	186.2
Common Units | Class B Common Units
Basic earnings per common unit:
Distributions declared									$ 1.5	$ 1.9	$ 3.1
Allocation percentage for undistributed earnings or distributions in excess of earnings									0.97%	0.97%	0.97%
Allocation of (distributions in excess of earnings) undistributed earnings									$ 0.0	$ 0.1	$ (0.1)
Net earnings allocated									1.5	2.0	3.0
General Partner's Interest
Basic earnings per common unit:
Distributions declared									$ 42.8	$ 63.1	$ 118.4
Allocation percentage for undistributed earnings or distributions in excess of earnings									27.70%	32.21%	38.36%
Allocation of (distributions in excess of earnings) undistributed earnings									$ (0.2)	$ 4.3	$ (0.7)
Net earnings allocable to General Partner (2)									42.6	67.4	117.7
General Partner and TLPH
Basic earnings per common unit:
Net earnings allocable to General Partner (2)									$ 1.5	$ 2.1	$ 3.1