Retirement Plans and Other Postretirement Benefits - Summary of Corporation's Pension Plan Asset Allocation (Detail)	Dec. 31, 2018	Dec. 31, 2017
Summary of corporation's pension plan asset allocation
Assets Category, Total	100.00%	100.00%
Equity Securities [Member]
Summary of corporation's pension plan asset allocation
Assets Category, Total	60.00%	64.00%
Debt Securities [Member]
Summary of corporation's pension plan asset allocation
Assets Category, Total	39.00%	35.00%
Other [Member]
Summary of corporation's pension plan asset allocation
Assets Category, Total	1.00%	1.00%