Weighted Average Asset Allocations for Retirement Plan (Detail)	Dec. 31, 2018	Dec. 31, 2017
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	57.00%	59.00%
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation	41.00%	39.00%
Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Weighted average asset allocation	2.00%	2.00%