Borrowings - Summary of Borrowings (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fixed-rate FHLB advances maturing:
Within 1 month	$ 3,060.8	$ 2,270.3
After 1 month but within 1 year	44.8	61.0
After 1 but within 2 years	6.8	54.2
After 2 but within 3 years	1.2	6.8
After 3 but within 4 years	0.6	1.3
After 4 but within 5 years	0.0	0.6
After 5 years	11.2	10.3
Total FHLB advances	3,125.4	2,404.5	$ 2,774.4
Federal funds purchased maturing:
Within 1 month	1,620.0	845.0
Total federal funds purchased	1,620.0	845.0	820.0
Customer repurchase agreements maturing:
Within 1 month	409.1	332.9
Total customer repurchase agreements	409.1	332.9	301.6
Other borrowings maturing:
Within 1 year	0.0	11.0
Total other borrowings	0.0	11.0	$ 207.8
Total borrowings	$ 5,154.5	$ 3,593.4
Fixed-rate FHLB advances maturing, weighted average rate
Within 1 month	1.79%	2.59%
After 1 month but within 1 year	1.79%	1.76%
After 1 but within 2 years	2.11%	1.75%
After 2 but within 3 years	0.51%	2.11%
After 3 but within 4 years	0.05%	0.52%
After 4 but within 5 years	0.00%	0.05%
After 5 years	1.50%	1.64%
Total FHLB advances	1.79%	2.54%
Federal funds purchased maturing, weighted average rate
Within 1 month	1.61%	2.53%
Total federal funds purchased	1.61%	2.53%
Customer repurchase agreements maturing:
Within 1 month	0.69%	0.61%
Total customer repurchase agreements	0.69%	0.61%
Other borrowings maturing:
Within 1 year	0.00%	2.40%
Total other borrowings	0.00%	2.40%
Total borrowings, weighted average rate	1.64%	2.36%