Retirement Plans and Other Postretirement Benefits - Summary of Corporation's Pension Plan Asset Allocation (Detail)	Dec. 31, 2020	Dec. 31, 2019
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%	61.00%
Debt Securities [Member]
Summary of corporation's pension plan asset allocation
Assets category, total	37.00%	38.00%
Other [Member]
Summary of corporation's pension plan asset allocation
Assets category, total	3.00%	1.00%