Net Earnings per Common Unit (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per Common Unit:
Net earnings	$ 44.1	$ 28.8	$ 98.7	$ 37.7	$ 79.2	$ 69.4	$ 99.3	$ 59.0	$ 209.3	$ 306.9	$ 370.0
Weighted-average Common Units outstanding									18,500,000	18,500,000	18,500,000
Net earnings per Common Unit	$ 1.85	$ 1.04	$ 3.22	$ 1.44	$ 2.49	$ 2.23	$ 3.31	$ 2.03	$ 7.56	$ 10.06	$ 12.07
Dilutive TNCLP units outstanding									0	0	0
General Partner's Interests
Basic earnings per Common Unit:
Net earnings									$ 67.4	$ 117.7	$ 143.1
Common Units | Class B Common Units
Basic earnings per Common Unit:
Net earnings									2.0	3.0	3.6
Common Units | Common Units
Basic earnings per Common Unit:
Net earnings									$ 139.9	$ 186.2	$ 223.3