Accounting Changes - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Expected increase in lease assets related to adoption of accounting guidance	$ 1,500
Expected increase in lease related liabilities related to adoption of accounting guidance	1,500
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of adoption of new accounting guidance	300
Accumulated Other Comprehensive Income (Loss) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of adoption of new accounting guidance	$ (300)
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Transfer from held-to-maturity to available-for-sale securities		$ 1,500