Postretirement Benefit Plans - Asset Allocations of Postretirement Benefit Plans (Detail)	Dec. 31, 2018
Minimum | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	0.00%
Minimum | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	15.00%
Minimum | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	10.00%
Minimum | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	0.00%
Minimum | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	0.00%
Minimum | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	0.00%
Minimum | Commodities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	0.00%
Maximum | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	20.00%
Maximum | Equity
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	65.00%
Maximum | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	60.00%
Maximum | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	15.00%
Maximum | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	10.00%
Maximum | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	20.00%
Maximum | Commodities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, target allocation percentage of assets	15.00%