Income Taxes - Component of Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Expected tax expense, at U.S. federal statutory income tax rate of 35% in 2017 and 21% in 2018	$ 3,800	$ 3,800
Rate differences on equity in earnings of Kronos	(1,200)	(1,600)
Nontaxable income	(100)	(200)
U.S. state income taxes and other, net	800	200
Income tax expense	$ 3,330	$ 2,179