Other Assets and Liabilities - Summary of Components of Other Assets (Parenthetical) (Detail) - USD ($) $ in Billions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Schedule Of Other Assets [Line Items]
Percentage of yield in subordinate risk retention interest	10.00%
2018 Single Asset Securitization [Member]
Schedule Of Other Assets [Line Items]
Single asset securitization principal amount	$ 1.0	$ 1.0	$ 1.0
Single asset securitization maximum maturity date	Jun. 09, 2025