Consolidated Statement of Stockholders' Equity (unaudited) - USD ($) $ in Millions	Total	Common Stock	Capital in Excess of Par Value	Retained Earnings	Accumulated Other Comprehensive Loss	Treasury Stock	Trinity Stockholders’ Equity	Noncontrolling Interest
Beginning balance (in shares) at Dec. 31, 2017		(150,900,000)				(100,000)
Beginning balance at Dec. 31, 2017	$ 4,858.0	$ 1.6	$ 482.5	$ 4,123.4	$ (104.8)	$ (1.6)	$ 4,501.1	$ 356.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	41.6			40.2			40.2	1.4
Other comprehensive (loss) income	1.9				1.5		1.5	0.4
Dividends, Common Stock, Cash	(19.4)			(19.4)			(19.4)
Stock-based compensation	6.2		6.2				6.2
Restricted shares, net, (in shares)						(100,000)
Shares repurchased, shares						(1,500,000)
Shares repurchased	(50.0)					$ (50.0)	(50.0)
Stock options exercised	0.1		0.1				0.1
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	2.0		3.9			$ (1.9)	2.0
Distributions to noncontrolling interest	(5.8)						0.0	(5.8)
Cumulative effect of new accounting principle	0.0			18.7	(18.7)		0.0
Ending balance (in shares) at Mar. 31, 2018		(150,900,000)				(1,700,000)
Ending balance at Mar. 31, 2018	4,834.6	$ 1.6	492.7	4,162.9	(122.0)	$ (53.5)	4,481.7	352.9
Beginning balance (in shares) at Dec. 31, 2017		(150,900,000)				(100,000)
Beginning balance at Dec. 31, 2017	4,858.0	$ 1.6	482.5	4,123.4	(104.8)	$ (1.6)	4,501.1	356.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	107.1
Other comprehensive (loss) income	1.4
Stock-based compensation	14.2
Shares repurchased, shares						(2,970,674)
Shares repurchased	(100.1)
Ending balance (in shares) at Jun. 30, 2018		(147,700,000)				(100,000)
Ending balance at Jun. 30, 2018	4,671.8	$ 1.5	237.2	4,207.4	(122.9)	$ (1.6)	4,321.6	350.2
Beginning balance (in shares) at Mar. 31, 2018		(150,900,000)				(1,700,000)
Beginning balance at Mar. 31, 2018	4,834.6	$ 1.6	492.7	4,162.9	(122.0)	$ (53.5)	4,481.7	352.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	65.5			64.1			64.1	1.4
Other comprehensive (loss) income	(0.5)				(0.9)		(0.9)	0.4
Dividends, Common Stock, Cash	(19.6)			(19.6)			(19.6)
Stock-based compensation	8.0		8.0				8.0
Restricted shares, net, (in shares)		200,000				(300,000)
Shares repurchased, shares						(1,451,171)
Shares repurchased	(50.1)					$ (50.1)	(50.1)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	(8.6)		4.2			$ (12.8)	(8.6)
Distributions to noncontrolling interest	(4.5)						0.0	(4.5)
Treasury Stock, Shares, Retired		(3,400,000)				3,400,000
Treasury Stock, Retired, Cost Method, Amount	0.0		(114.8)			$ 114.8	0.0
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt	(152.9)		(152.9)				(152.9)
Stockholders' equity, other	(0.1)	$ (0.1)					(0.1)
Ending balance (in shares) at Jun. 30, 2018		(147,700,000)				(100,000)
Ending balance at Jun. 30, 2018	4,671.8	$ 1.5	237.2	4,207.4	(122.9)	$ (1.6)	4,321.6	350.2
Beginning balance (in shares) at Dec. 31, 2018		(133,300,000)				(100,000)
Beginning balance at Dec. 31, 2018	2,562.0	$ 1.3	1.2	2,326.1	(116.8)	$ (1.0)	2,210.8	351.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30.1			30.6			30.6	(0.5)
Other comprehensive (loss) income	(3.8)				(4.1)		(4.1)	0.3
Dividends, Common Stock, Cash	(22.3)			(22.3)			(22.3)
Stock-based compensation	5.5		5.5				5.5
Shares repurchased, shares						(3,500,000)
Shares repurchased	(19.0)		70.0			$ (89.0)	(19.0)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	(0.5)		0.6			$ (1.1)	(0.5)
Distributions to noncontrolling interest	(0.4)						0.0	(0.4)
Cumulative effect of new accounting principle	13.7			13.7			13.7
Stockholders' equity, other	(0.2)			(0.2)			(0.2)
Ending balance (in shares) at Mar. 31, 2019		(133,300,000)				(3,600,000)
Ending balance at Mar. 31, 2019	2,565.1	$ 1.3	77.3	2,347.9	(120.9)	$ (91.1)	2,214.5	350.6
Beginning balance (in shares) at Dec. 31, 2018		(133,300,000)				(100,000)
Beginning balance at Dec. 31, 2018	2,562.0	$ 1.3	$ 1.2	2,326.1	(116.8)	$ (1.0)	2,210.8	351.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	66.9
Other comprehensive (loss) income	(10.0)							0.6
Stock-based compensation	13.0
Shares repurchased, shares			(2,607,172)			(5,607,003)
Shares repurchased			$ (70.0)			$ (133.0)
Ending balance (in shares) at Jun. 30, 2019		(127,900,000)				(100,000)
Ending balance at Jun. 30, 2019	2,529.6	$ 1.3	0.0	2,305.8	(127.4)	$ (0.9)	2,178.8	350.8
Beginning balance (in shares) at Mar. 31, 2019		(133,300,000)				(3,600,000)
Beginning balance at Mar. 31, 2019	2,565.1	$ 1.3	77.3	2,347.9	(120.9)	$ (91.1)	2,214.5	350.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36.8			36.4			36.4	0.4
Other comprehensive (loss) income	(6.2)				(6.5)		(6.5)	0.3
Dividends, Common Stock, Cash	(21.6)			(21.6)			(21.6)
Stock-based compensation	7.5		7.5				7.5
Restricted shares, net, (in shares)		700,000				(500,000)
Shares repurchased, shares						(2,133,116)
Shares repurchased	(44.0)					$ (44.0)	(44.0)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	(7.5)		1.0			$ (8.5)	(7.5)
Distributions to noncontrolling interest	(0.5)						0.0	(0.5)
Treasury Stock, Shares, Retired		(6,100,000)				6,100,000
Treasury Stock, Retired, Cost Method, Amount	0.0		(85.8)	(56.9)		$ (142.7)	0.0
Ending balance (in shares) at Jun. 30, 2019		(127,900,000)				(100,000)
Ending balance at Jun. 30, 2019	$ 2,529.6	$ 1.3	$ 0.0	$ 2,305.8	$ (127.4)	$ (0.9)	$ 2,178.8	$ 350.8