Benefit Plans - Weighted Average Discount Rate (Detail) - Pension Plan	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.12%	4.33%	4.02%
Expected long-term return on plan assets	7.50%	7.50%	7.50%