Borrowings - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Schedule Of Borrowings [Line Items]
Junior subordinated debentures	$ 515
Trust preferred capital securities	0.00%
Debt Maturity, Start Year	Jan. 01, 2027
Debt Maturity, End Year	Dec. 31, 2033
Agreements to repurchase securities	$ 1,900	$ 1,900
Collateral posted	$ 2,100	2,000
Latest year of repurchase for securities sold under agreements to repurchase	2020
Junior Subordinated Debt [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures redeemed		$ 323