Consolidated Statements of Stockholders' Equity - 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 at Dec. 31, 2014	$ 3,397.4	$ 155.7	$ 463.2	$ 2,489.9	$ (111.9)	$ (1.0)	$ 2,995.9	$ 401.5
Beginning balance, shares at Dec. 31, 2014		155,700,000				(100,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	826.0			796.5			796.5	29.5
Other comprehensive income	(0.5)				(3.5)		(3.5)	3.0
Cash dividends on common stock	(66.1)			(66.1)			(66.1)
Restricted shares, net	33.6	$ 0.0	64.4			$ (30.8)	33.6
Restricted shares, net, shares		2,100,000				(1,100,000)
Shares repurchased	(115.0)					$ (115.0)	(115.0)
Shares repurchased, shares						(3,900,000)
Stock options exercised	0.4	$ 0.0	0.4			$ 0.0	0.4
Stock options exercised, shares		100,000				0
Excess tax benefits from stock-based compensation	12.1		12.1				12.1
Distributions to noncontrolling interest	(39.2)						0.0	(39.2)
Retirement of treasury stock	0.0	$ 0.0	(145.8)			$ 145.8	0.0
Retirement of treasury stock, shares		(5,000,000)				5,000,000
Change in par value of common stock		$ (154.2)	154.2
Ending balance at Dec. 31, 2015	4,048.7	$ 1.5	548.5	3,220.3	(115.4)	$ (1.0)	3,653.9	394.8
Ending balance, shares at Dec. 31, 2015		152,900,000				(100,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	364.7			343.6			343.6	21.1
Other comprehensive income	5.0				1.9		1.9	3.1
Cash dividends on common stock	(66.6)			(66.6)			(66.6)
Restricted shares, net	25.0	$ 0.1	46.2			$ (21.3)	25.0
Restricted shares, net, shares		2,600,000				(1,200,000)
Shares repurchased	$ (34.7)					$ (34.7)	(34.7)
Shares repurchased, shares	2,070,600					(2,100,000)
Excess net tax deficiency from stock-based compensation	$ (4.5)		(4.5)				(4.5)
Distributions to noncontrolling interest	(26.4)						0.0	(26.4)
Retirement of treasury stock	0.0	$ 0.0	(56.0)			$ 56.0	0.0
Retirement of treasury stock, shares		(3,300,000)				3,300,000
Other	(0.1)		0.4	0.0		$ (0.5)	(0.1)
Ending balance at Dec. 31, 2016	4,311.1	$ 1.6	534.6	3,497.3	(113.5)	$ (1.5)	3,918.5	392.6
Ending balance, shares at Dec. 31, 2016		152,200,000				(100,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	713.6			702.5			702.5	11.1
Other comprehensive income	11.3				8.7		8.7	2.6
Cash dividends on common stock	(75.8)			(75.8)			(75.8)
Restricted shares, net	16.7	$ 0.0	35.2			$ (18.5)	16.7
Restricted shares, net, shares		1,700,000				(700,000)
Shares repurchased	$ (85.4)					$ (85.4)	(85.4)
Shares repurchased, shares	2,825,400					(2,800,000)
Stock options exercised	$ 0.2	$ 0.0	0.2			$ 0.0	0.2
Stock options exercised, shares	26,450	0				0
Distributions to noncontrolling interest	$ (48.7)						0.0	(48.7)
Retirement of treasury stock	0.0	$ 0.0	(103.8)			$ 103.8	0.0
Stock issued for acquisition (shares)		500,000
Stock issued for acquisition (value)	14.7		14.7				14.7
Retirement of treasury stock, shares		(3,500,000)				3,500,000
Other	0.3		1.6	(0.6)		$ 0.0	1.0	(0.7)
Ending balance at Dec. 31, 2017	$ 4,858.0	$ 1.6	$ 482.5	$ 4,123.4	$ (104.8)	$ (1.6)	$ 4,501.1	$ 356.9
Ending balance, shares at Dec. 31, 2017		150,900,000				(100,000)