Target and Weighted Average Allocation for Pension and OPB Plans by Asset Category (Detail)	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	100.00%	100.00%
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	100.00%	100.00%
Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	4.00%	4.00%
Cash Equivalents [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	7.00%	7.00%
Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	13.00%	29.00%
Equity Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	30.00%	34.00%
Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	83.00%	67.00%
Debt Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage	63.00%	59.00%
Minimum [Member] | Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum [Member] | Cash Equivalents [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%
Minimum [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum [Member] | Equity Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	20.00%
Minimum [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	75.00%
Minimum [Member] | Debt Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	50.00%
Maximum [Member] | Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	10.00%
Maximum [Member] | Cash Equivalents [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	15.00%
Maximum [Member] | Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	25.00%
Maximum [Member] | Equity Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	40.00%
Maximum [Member] | Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	100.00%
Maximum [Member] | Debt Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	70.00%