Derivative Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Realized and unrealized gains (losses) on derivatives classified as cash flow hedges recorded in other comprehensive income (loss)			$ 204	$ 71
Non-derivative debt instruments designated as net investment hedges			1,200	$ 1,200
Fair value of derivatives under collateral agreements in a net liability position			443
Collateral posted by company netted against net liability position			369
Forward commitments to sell mortgage loans			4,000
Hedged mortgage loans held for sale			2,200
Unfunded mortgage loan commitments			$ 2,300
Scenario, Forecast [Member]
Derivative [Line Items]
Estimated gain to be reclassified from other comprehensive income (loss) into earnings	$ 11	$ 76