Income Taxes - Component of Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Expected tax expense (benefit), at U.S. federal statutory income tax rate of 35% in 2017 and 21% in 2018	$ (11,500)	$ 21,700	$ (7,700)	$ 25,500
Rate differences on equity in earnings of Kronos	(1,200)	(1,300)	(2,400)	(2,900)
Nontaxable income	(100)		(200)	(200)
U.S. state income taxes and other, net	200	(100)	1,000	100
Income tax expense (benefit)	$ (12,588)	$ 20,278	$ (9,258)	$ 22,457