Target and Weighted Average Allocation for Pension and OPB Plans by Asset Category (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%	100.00%
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%	100.00%
Cash Equivalents [Member] | Pension Plan [Member]
Defined Benefit Plan, Target Allocation Percentage	0-10%
Defined Benefit Plan, Actual Plan Asset Allocations	4.00%	3.00%
Cash Equivalents [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Target Allocation Percentage	5-15%
Defined Benefit Plan, Actual Plan Asset Allocations	7.00%	10.00%
Equity Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Target Allocation Percentage	0-40%
Defined Benefit Plan, Actual Plan Asset Allocations	29.00%	47.00%
Equity Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Target Allocation Percentage	20-40%
Defined Benefit Plan, Actual Plan Asset Allocations	34.00%	30.00%
Debt Securities [Member] | Pension Plan [Member]
Defined Benefit Plan, Target Allocation Percentage	40-100%
Defined Benefit Plan, Actual Plan Asset Allocations	67.00%	50.00%
Debt Securities [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Target Allocation Percentage	50-70%
Defined Benefit Plan, Actual Plan Asset Allocations	59.00%	60.00%