Loans and Leases - Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable
Net deferred loan origination fees	$ 30,100		$ 30,100		$ 30,200
Net discount	114,300		114,300		136,800
Deposit liabilities reclassified as loans receivable	5,600		5,600		9,200
Loans with carrying value pledged to secure public deposits and other borrowings	8,000,000		8,000,000		7,600,000
Acquired Loans
Accounts, Notes, Loans and Financing Receivable
Net discount	71,600		71,600		81,600
Sabadell United | Non-US
Accounts, Notes, Loans and Financing Receivable
Covered loans acquired	200,900		200,900		$ 202,600
TDRs Occurring during the Period | TDRs
Accounts, Notes, Loans and Financing Receivable
TDRs during period	$ 20,652	$ 20,108	37,409	$ 46,691
TDRs during period on accrual status			13,900	20,600
TDRs during period on non-accrual status			$ 23,500	$ 26,100