Income Taxes - Income Tax Benefits in Shareholders' Equity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax benefits arising from compensation expense for tax purposes in excess of amounts recognized for financial statement purposes	$ 0	$ 13,661	$ 23,311