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
Common Stock, Shares, Issued		(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 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 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	135.4
Other comprehensive (loss) income	3.5
Stock-based compensation	21.8
Shares repurchased, shares						(4,327,158)
Shares repurchased	(150.1)
Ending balance at Sep. 30, 2018	4,643.4	$ 1.5	250.2	4,216.1	(121.1)	$ (54.4)	4,292.3	351.1
Common Stock, Shares, Issued		(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						(1,500,000)
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 at Jun. 30, 2018	4,671.8	$ 1.5	237.2	4,207.4	(122.9)	$ (1.6)	4,321.6	350.2
Common Stock, Shares, Issued		(147,700,000)				(100,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	28.3			27.7			27.7	0.6
Other comprehensive (loss) income	2.1				1.8		1.8	0.3
Dividends, Common Stock, Cash	(19.0)			(19.0)			(19.0)
Stock-based compensation	7.6		7.6				7.6
Restricted shares, net, (in shares)						0
Shares repurchased, shares						(1,356,484)
Shares repurchased	(50.0)					$ (50.0)	(50.0)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	2.6		5.4			(2.8)	2.6
Ending balance at Sep. 30, 2018	4,643.4	$ 1.5	250.2	4,216.1	(121.1)	$ (54.4)	4,292.3	351.1
Common Stock, Shares, Issued		(147,700,000)				(1,500,000)
Common Stock, Shares, Issued		(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 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 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	114.6
Other comprehensive (loss) income	(12.7)							1.0
Stock-based compensation	21.4
Shares repurchased, shares			(2,607,172)			(10,778,492)
Shares repurchased			$ (70.0)			$ (233.9)
Ending balance at Sep. 30, 2019	2,459.3	$ 1.3	0.0	2,342.1	(130.5)	$ (102.3)	2,110.6	348.7
Common Stock, Shares, Issued		(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						(2,100,000)
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 at Jun. 30, 2019	2,529.6	$ 1.3	0.0	2,305.8	(127.4)	$ (0.9)	2,178.8	350.8
Common Stock, Shares, Issued		(127,900,000)				(100,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47.7			49.0			49.0	(1.3)
Other comprehensive (loss) income	(2.7)				(3.1)		(3.1)	0.4
Dividends, Common Stock, Cash	(21.3)			(21.3)			(21.3)
Stock-based compensation	8.4		8.4				8.4
Restricted shares, net, (in shares)		0				0
Shares repurchased, shares						(5,171,489)
Shares repurchased	(100.9)		(8.6)	8.6		$ (100.9)	(100.9)
Stock Issued During Period, Value, Restricted Stock Award, Net of Forfeitures	(0.3)		0.2			(0.5)	(0.3)
Distributions to noncontrolling interest	(1.2)						0.0	(1.2)
Ending balance at Sep. 30, 2019	$ 2,459.3	$ 1.3	$ 0.0	$ 2,342.1	$ (130.5)	$ (102.3)	$ 2,110.6	$ 348.7
Common Stock, Shares, Issued		(127,900,000)				(5,300,000)