Charges and Credits - Summary of Charges and Credits (Detail) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Sep. 30, 2017
Charges And Credits [Line Items]
Charges and credits, Pretax				$ 723,000,000
Charges and credits, Tax				44,000,000
Charges and credits, Noncontrolling Interests				12,000,000
Charges and credits, Net	$ 0	$ 0	$ 50,000,000	667,000,000
Promissory Note Fair Value Adjustment and Other
Charges And Credits [Line Items]
Charges and credits, Pretax				510,000,000
Charges and credits, Noncontrolling Interests				12,000,000
Charges and credits, Net			$ 460,000,000	498,000,000
Merger and Integration
Charges And Credits [Line Items]
Charges and credits, Pretax				213,000,000
Charges and credits, Tax				44,000,000
Charges and credits, Net				$ 169,000,000