Other Borrowings - Company's Borrowings (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Jun. 16, 2016
Short Term
Federal Funds Purchased	$ 675,000,000	$ 0
FHLB advances	0	5,000,000.0
Short-term Debt	726,900,000	21,000,000.0
Customer repurchase agreements	16,600,000	16,000,000.0
Long Term
Other Short-term Borrowings	35,300,000	0
Senior Notes	317,900,000	0
Notes Payable	457,100,000	0
Long-term Debt	775,000,000.0	0
Other borrowings	1,501,900,000	$ 21,000,000.0
Credit Linked Notes Maturing December 2024 [Member]
Long Term
Debt Instrument, Face Amount	242,000,000
Debt Issuance Cost	2,900,000
Credit Linked Notes Maturing July 2059
Long Term
Debt Instrument, Face Amount	227,600,000
Debt Issuance Cost	$ 2,800,000
Subordinated Debt [Member] | Subordinated Debentures Maturing June 2031
Long Term
Debt Instrument, Interest Rate, Stated Percentage	3.00%
Debt Instrument, Face Amount	$ 600,000,000.0
Debt Issuance Cost	$ 8,100,000
Subordinated Debt [Member] | Subordinated Debentures Maturing June 2030 [Member]
Long Term
Debt Instrument, Interest Rate, Stated Percentage	5.25%	5.25%
Debt Instrument, Face Amount	$ 225,000,000.0	$ 225,000,000.0
Debt Issuance Cost	3,100,000	$ 3,100,000
Subordinated Debt [Member] | Subordinated Debentures Maturing July 2056 [Member]
Long Term
Debt Instrument, Interest Rate, Stated Percentage		6.25%	6.25%
Debt Instrument, Face Amount		$ 175,000,000.0	$ 175,000,000
Debt Issuance Cost		$ 5,500,000
Subordinated Debt [Member] | Subordinated Debentures Maturing July 2025 [Member]
Long Term
Debt Instrument, Interest Rate, Stated Percentage		3.44%
Debt Instrument, Face Amount		$ 75,000,000.0
Debt Issuance Cost		1,800,000
Payments on long-term debt	75,000,000
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 7,800,000,000	$ 4,000,000,000