Gross Realized Gains and Losses on Sales and OTTI on Securities Available for Sale (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Available-for-sale Securities, Gross Realized Gains		$ 0.0	$ 0.0
Available-for-sale Securities	[1]	31,127.0		$ 30,672.0
Available-for-sale Securities, Gross Realized Losses		0.0	0.0
Gain (Loss) on Sale of Securities, Net		0.0	$ 0.0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment		$ 0.0

[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.