Income Taxes - Component of Income Taxes Expenses (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components of income tax expense (benefit):
Currently payable (receivable)	$ 800,000		$ (15,000,000)
Deferred income taxes (benefit)	4,200,000	(41,900,000)	34,900,000
Income tax expense (benefit)	$ 5,003,000	$ (41,911,000)	$ 19,933,000