LOAN PORTFOLIO- Corporation's Aging of Purchased Credit Impaired Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 8,996,816	$ 8,858,123	$ 8,695,890
Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	3,126,562	3,163,208	3,267,868
Commercial Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	3,854,962	3,750,202
Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	1,558,724	1,522,662	1,552,503
Financing Receivables 30 To 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	6,000	5,100
PCI loans
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	144,443	146,640	155,281	$ 158,174
PCI loans | Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	140,979	143,176	151,067
PCI loans | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	3,464	3,464	$ 4,214
PCI loans | Financing Receivables 30 To 59 Days Past Due [Member] | Residential Portfolio Segment [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	12,800	11,600
PCI loans | Financing Receivables 30 To 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 0	$ 0