Retirement Plans and Other Postretirement Benefits - Summary of Corporation's Pension Plan Asset Allocation (Detail)	Dec. 31, 2019	Dec. 31, 2018
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	61.00%	60.00%
Debt Securities [Member]
Summary of corporation's pension plan asset allocation
Assets category, total	38.00%	39.00%
Other [Member]
Summary of corporation's pension plan asset allocation
Assets category, total	1.00%	1.00%