Actual Investment Allocations (Detail)		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation			100.00%	100.00%	100.00%
Pension Plans | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation			49.00%	46.00%	46.00%
Pension Plans | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation			34.00%	37.00%	37.00%
Pension Plans | Equity And Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation	[1]		6.00%	7.00%	6.00%
Pension Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation	[2]		1.00%
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation			100.00%	100.00%	100.00%
Other Postretirement Benefit Plans, Defined Benefit | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation			56.00%	55.00%	53.00%
Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation			42.00%	43.00%	47.00%
Other Postretirement Benefit Plans, Defined Benefit | Other
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation	[3]		2.00%	2.00%	0.00%
Real Estate Funds | Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Actual Allocation			10.00%	10.00%	11.00%
Scenario Forecast | Pension Plans | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target Allocation		47.00%
Scenario Forecast | Pension Plans | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Target Allocation		37.00%
Scenario Forecast | Pension Plans | Equity And Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	[1]	6.00%
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Target Allocation		55.00%
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Target Allocation		45.00%
Scenario Forecast | Other Postretirement Benefit Plans, Defined Benefit | Other
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	[3]	0.00%
Scenario Forecast | Real Estate Funds | Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Target Allocation		10.00%

[1] Represents investments in an asset allocation fund. This fund invests in both equity and debt securities.

[2] Represents investments being held in cash equivalents as of December 31, 2017 pending payment of benefits.

[3] Represents investments being held in cash equivalents as of December 31, 2017 and 2016 pending transfer into debt and equity funds.