Charges and Credits - Summary of Charges and Credits (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2017
Charges And Credits [Line Items]
Charges and credits, Pretax			$ 674,000,000
Charges and credits, Tax			31,000,000
Charges and credits, Noncontrolling Interests			12,000,000
Charges and credits, Net	$ 0	$ 50,000,000	631,000,000
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			164,000,000
Charges and credits, Tax			31,000,000
Charges and credits, Net			$ 133,000,000