Pension and Other Postretirement Benefits - Schedule of Pension and Postretirement Plans Investment Policy and Weighted Average Asset Allocations (Detail)	Dec. 31, 2017	Dec. 31, 2016
Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	20.00%
Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations		55.00%
Fixed Income Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	25.00%
Fixed Income Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations		55.00%
Other Investments [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	15.00%
Other Investments [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations		55.00%
Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	100.00%	100.00%
Plan Assets [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	40.00%	37.00%
Plan Assets [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	35.00%	36.00%
Plan Assets [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	25.00%	27.00%