Derivative Instruments - Additional Information (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Realized and unrealized gains (losses) on derivatives classified as cash flow hedges recorded in other comprehensive income (loss)			$ 51,000,000	$ 55,000,000
Non-derivative debt instruments designated as net investment hedges			1,100,000,000	$ 0
Fair value of derivatives under collateral agreements in a net liability position			616,000,000
Collateral posted by company netted against net liability position			576,000,000
Forward commitments to sell mortgage loans			5,500,000,000
Hedged mortgage loans held for sale			2,500,000,000
Unfunded mortgage loan commitments			$ 3,900,000,000
Scenario, Forecast [Member]
Derivative [Line Items]
Estimated loss to be reclassified from other comprehensive income (loss) into earnings	$ 4,000,000	$ 5,000,000