Loans and Leases - Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable
Net deferred loan origination fees	$ 37,800		$ 37,800		$ 30,200
Net discount	97,800		97,800		136,800
Deposit liabilities reclassified as loans receivable	7,000		7,000		9,200
Loans with carrying value pledged to secure public deposits and other borrowings	8,500,000		8,500,000		7,600,000
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Net discount	61,800		61,800		81,600
Sabadell United | Non-US
Accounts, Notes, Loans and Financing Receivable
Covered loans acquired	194,600		194,600		$ 202,600
TDRs Occurring during the Period | TDRs
Accounts, Notes, Loans and Financing Receivable
TDRs during period	$ 28,738	$ 6,537	61,028	$ 45,954
TDRs during period on accrual status			25,500	20,200
TDRs during period on non-accrual status			$ 35,500	$ 25,800