Benefit and Incentive Compensation Plans and Other - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Contribution to Savings Plan	$ 0.2	$ 0.2	$ 0.2
Time Based Restricted Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based award vesting period	5 years
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based award vesting period	5 years