Securities Available for Sale - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 0.0	$ 0.0
Available-for-sale Securities, Gross Realized Losses		0.0	$ 0.0
Available-for-sale Securities Pledged as Collateral		3,300.0		$ 2,000.0
Available-for-sale Securities	[1]	$ 31,127.0		$ 30,672.0

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $100 million and $0 million at March 31, 2017 and December 31, 2016, respectively.