Weighted Average Asset Allocations for Retirement Plan (Detail)	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation	100.00%	100.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation	59.00%	56.00%
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation	39.00%	43.00%
Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation	2.00%	1.00%