Pension and Other Postretirement Benefits - Total Expenses Recognized to All Pension and Other Postretirement Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Shared Plans - Cost allocation	$ 69	$ 65	$ 57
Total expense related to all pension and other post retirement benefits	139	85	24
Pension Benefits [Member]
Defined Contribution Plan Disclosure [Line Items]
Cumulative Direct Plans - Net periodic benefit cost	561	119	83
Shared Plans - Multiemployer contribution expense			28
Shared Plans - Cost allocation			25
Total expense related to all pension and other post retirement benefits	561	119	136
Other Postretirement Benefits [Member]
Defined Contribution Plan Disclosure [Line Items]
Cumulative Direct Plans - Net periodic benefit cost	(32)	50	21
Shared Plans - Multiemployer contribution expense			12
Shared Plans - Cost allocation			8
Total expense related to all pension and other post retirement benefits	$ (32)	$ 50	$ 41