Net Earnings per Common Unit (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net earnings per limited partner common unit
Net earnings	$ 22.8	$ 28.8	$ 104.5	$ 165.2
(Distributions in excess of earnings) undistributed earnings	$ (11.0)	$ (20.2)	$ 8.5	$ (2.1)
Weighted-average common units outstanding	18,500,000	18,500,000	18,500,000	18,500,000
Net earnings per Common Unit (in dollars per unit)	$ 0.92	$ 1.04	$ 4.49	$ 5.71
Dilutive TNCLP units outstanding	0	0	0	0
General Partner
Net earnings per limited partner common unit
Net earnings	$ 5.6	$ 9.3	$ 20.3	$ 58.0
Distributions declared	8.4	15.8	22.5	57.3
Allocation of (distributions in excess of earnings) undistributed earnings	(2.8)	(6.5)	(2.2)	0.7
Net earnings allocable to General Partner	$ 5.6	$ 9.3	20.3	58.0
Allocation percentage for undistributed earnings or distributions in excess of earnings	24.64%	32.22%
Common Units \| Class B Common Units
Net earnings per limited partner common unit
Net earnings	$ 0.2	$ 0.3	1.0	1.6
Distributions declared	0.3	0.5	0.9	1.6
Allocation of (distributions in excess of earnings) undistributed earnings	(0.1)	(0.2)	0.1	0.0
Net earnings allocated	$ 0.2	$ 0.3	1.0	1.6
Allocation percentage for undistributed earnings or distributions in excess of earnings	0.97%	0.97%
Common Units \| Common Units
Net earnings per limited partner common unit
Net earnings	$ 17.0	$ 19.2	83.2	105.6
Distributions declared	25.1	32.7	72.6	108.4
Allocation of (distributions in excess of earnings) undistributed earnings	(8.1)	(13.5)	10.6	(2.8)
Net earnings allocated	$ 17.0	$ 19.2	83.2	105.6
Allocation percentage for undistributed earnings or distributions in excess of earnings	74.39%	66.81%
General Partner and TLPH
Net earnings per limited partner common unit
Net earnings allocable to General Partner	$ 0.2	$ 0.3	$ 1.0	$ 1.7

Undistributed Earnings (Loss) Allocated to Participating Securities, Percent

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The total amount of dividends declared in the period for each class of stock and the contractual amount of dividends (or interest on participating income bonds) that must be paid for the period (for example, unpaid cumulative dividends). Dividends declared in the current period do not include dividends declared in respect of prior-period unpaid cumulative dividends. Preferred dividends that are cumulative only if earned are deducted only to the extent that they are earned.

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Amount, before tax, of income (loss) attributable to parent. Includes, but is not limited to, income (loss) from continuing operations, discontinued operations and equity method investments.

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Aggregate amount of net income allocated to general partners.

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Aggregate amount of net income allocated to limited partners.

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Per unit of ownership amount after tax of income (loss) available to outstanding limited partnership (LP) unit-holder.

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The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed.

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Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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Weighted average number of limited partnership units outstanding determined by relating the portion of time within a reporting period that limited partnership units have been outstanding to the total time in that period.

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