Accounting Changes - Additional Information (Detail) $ in Millions	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in retained earnings related to adoption of accounting guidance	$ 300
Decrease in accumulated other comprehensive income related to adoption of 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