Incentive Plans - Schedule of Issuance of RSUs and Charges to Retained Earnings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of RSUs issued	985,584	1,085,354	693,714
Charges to retained earnings, net of estimated forfeitures	$ 41,746	$ 37,284	$ 34,255