Offsetting of Assets and Liabilities - Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	$ 1,999.7	$ 2,203.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,357.1	1,860.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	642.6	343.3
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	12.7	2.1
Net Amount	629.9	341.2
Total Derivatives Not Subject to a Master Netting Arrangement
Total Derivatives Not Subject to a Master Netting Arrangement	563.8	450.8
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	2.7	2.5
Net Amount	561.1	448.3
Total Derivatives
GROSS RECOGNIZED ASSETS	2,563.5	2,654.1
Derivative Assets	1,206.4	794.1
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	15.4	4.6
Net Amount	1,191.0	789.5
Securities Purchased under Agreements to Resell
GROSS RECOGNIZED ASSETS	1,031.2	1,303.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1,031.2	1,303.3
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	1,031.2	1,303.3
Other Assets (excluding Derivative Assets)	4,600.0	3,900.0
Federal funds sold	134.0	21.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	1,902.3	2,106.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,175.3	1,397.7
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	727.0	708.6
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	12.7	2.1
Net Amount	714.3	706.5
Interest Rate Swaps | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	71.6	86.9
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	19.0	14.2
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	52.6	72.7
Net Amount	52.6	72.7
Interest Rate Swaps | Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	24.5	10.1
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	24.4	10.1
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	0.1	0.0
Net Amount	0.1	0.0
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	1.3
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1.3
Net Amount	1.3
Cross Product Netting Adjustment
Total Derivatives Subject to a Master Netting Arrangement
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	3.9	10.4
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	$ 134.5	$ 427.6