Consolidated Statement of Stockholders' Equity (unaudited) - USD ($) $ in Millions		Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Capital in Excess of Par Value	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Loss	Treasury Stock	Trinity Stockholders’ Equity	Trinity Stockholders’ Equity Cumulative Effect, Period of Adoption, Adjustment	Noncontrolling Interest
$0.01 Par Value		$ 0.01
Common Stock, Shares, Issued				(119,700,000)					(100,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Beginning Balance at Dec. 31, 2019		$ 2,378.9		$ 1.2	$ 0.0	$ 2,182.9		$ (153.1)	$ (0.9)	$ 2,030.1		$ 348.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		162.3				161.7				161.7		0.6
Other comprehensive income		(23.2)						(23.5)		(23.5)		0.3
Cash dividends declared on common stock (1)	[1]	(24.6)				(24.6)				(24.6)
Stock-based compensation expense		7.3			7.3					7.3
Shares repurchased									(1,900,000)
Shares repurchased		(35.4)							$ (35.4)	(35.4)
Settlement of share-based awards, net		(0.2)			0.7				(0.9)	(0.2)
Cumulative effect of adopting new accounting standard			$ 0.5				$ 0.5				$ 0.5
Other		(0.5)				0.5				0.5
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance at Mar. 31, 2020		$ 2,466.1		$ 1.2	8.0	2,321.0		(176.6)	$ (37.2)	2,116.4		349.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Declared		$ 0.19
$0.01 Par Value		0.01
Common Stock, Shares, Issued				(119,700,000)					(2,000,000.0)
$0.01 Par Value		$ 0.01
Common Stock, Shares, Issued				(111,200,000)					100,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Beginning Balance at Dec. 31, 2020		$ 2,016.0		$ 1.1	0.0	1,769.4		(30.9)	$ (0.8)	1,738.8		277.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		1.3				3.3				3.3		(2.0)
Other comprehensive income		5.4						5.1		5.1		0.3
Cash dividends declared on common stock (1)		(23.3)				(23.3)				(23.3)
Stock-based compensation expense		5.4			5.4					5.4
Shares repurchased									(1,291,860)
Shares repurchased		(36.8)							$ (36.8)	(36.8)
Settlement of share-based awards, net		(0.1)			0.7				(0.8)	(0.1)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance at Mar. 31, 2021		$ 1,967.9		$ 1.1	$ 6.1	$ 1,749.4		$ (25.8)	$ (38.4)	$ 1,692.4		$ 275.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Declared		$ 0.21
$0.01 Par Value		$ 0.01
Common Stock, Shares, Issued				(111,200,000)					1,400,000

[1] Dividends of $0.21 and $0.19 per common share for the three months ended March 31, 2021 and 2020, respectively.