Income Taxes - Component of Income Taxes Expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of income tax benefit:
Currently payable (receivable)	$ (100)	$ 1,500	$ 100
Deferred income tax benefit	(5,500)	(4,300)	(28,700)
Income tax benefit	$ (5,634)	$ (2,777)	$ (28,611)