Borrowings - Information Concerning Parent Company Borrowings (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
FHLB advances:
Balance at year end	$ 3,125.4	$ 2,404.5	$ 2,774.4
Average outstanding during the year	2,098.0	2,653.6	2,677.5
Maximum outstanding at any month end	$ 3,125.5	$ 3,510.1	$ 3,130.8
Average interest rate during the year	2.39%	2.05%	1.17%
Federal funds purchased:
Balance at year end	$ 1,620.0	$ 845.0	$ 820.0
Average outstanding during the year	1,127.5	682.2	643.5
Maximum outstanding at any month end	$ 1,665.0	$ 855.0	$ 820.0
Average interest rate during the year	2.18%	2.00%	1.11%
Customer repurchase agreements:
Balance at year end	$ 409.1	$ 332.9	$ 301.6
Average outstanding during the year	296.6	252.7	311.0
Maximum outstanding at any month end	$ 409.1	$ 332.9	$ 331.6
Average interest rate during the year	0.75%	0.40%	0.19%
Other borrowings:
Balance at year end	$ 0.0	$ 11.0	$ 207.8
Average outstanding during the year	3.3	104.5	132.0
Maximum outstanding at any month end	$ 10.4	$ 207.6	$ 237.4
Average interest rate during the year	1.87%	1.66%	1.60%
Securities Sold under Agreements to Repurchase [Member]
Customer repurchase agreements:
Carrying amount of collateral securities at year end	$ 418.0	$ 342.3	$ 307.7