Schedule of Plan Assets Category (Detail)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Asset Allocation	100.00%	100.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Asset Allocation	23.00%	23.00%
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Asset Allocation	75.00%	75.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Asset Allocation	2.00%	2.00%