Business Combinations, Dispositions and Related Transactions - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2017	Nov. 30, 2015	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Impairment of long lived assets held for use						$ 170.6
Contract termination fee received	$ 4.0
Waste Management
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Impairment of long lived assets held for use				$ 170.6
Waste Management | Discontinued Operations | Scenario, Forecast
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Expected pre-tax gain from disposal of discontinued operations			$ 57.0
Kronos Worldwide, Inc.
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Repurchase of common stock									2,000,000
Repurchased shares						0	0	0
Shares available for purchase						1,950,000
CompX | Class A
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Shares available for purchase						678,000
WCS
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Impairment of long lived assets held for use					$ 170.6
Property and equipment, salvage value	$ 5.7			$ 5.7	5.7
Expenses related to sale of business including legal fees						$ 8.7	$ 5.8
WCS | Net Property and Equipment
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Impairment of long lived assets held for use					127.5
WCS | Disposal Site Operating Permits, Net
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Impairment of long lived assets held for use					42.0
WCS | Other Assets
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Impairment of long lived assets held for use					$ 1.1
WCS | Divestiture Terms Pending
Business Combinations Discontinued Operations And Related Transactions [Line Items]
Proceeds from sale of subsidiary		$ 270.0