LOAN PORTFOLIO - Corporation's Aging of Loans Held for Investment Portfolio (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	$ 120,204	$ 129,083
Period Of Residential Mortgage Loan That Are No Longer Accruing Interest	1 year 3 months
Loans and Leases Receivable, Gross	$ 8,996,816	8,858,123	$ 8,695,890
Residential Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross	3,126,562	3,163,208	3,267,868
Residential Portfolio Segment [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	15,453	14,564
Loans and Leases Receivable, Gross	$ 2,984,907	3,016,003
Residential Portfolio Segment [Member] | Financing Receivables 30 To 59 Days Past Due [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Impaired, Description	According to the Corporation's delinquency policy and consistent with the instructions for the preparation of the Consolidated Financial Statements for Bank Holding Companies (FR Y-9C) required by the Federal Reserve Board, residential mortgage, commercial mortgage, and construction loans are considered past due when the borrower is in arrears on two or more monthly payments.
Loans and Leases Receivable, Gross	$ 106,600	101,400
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross	3,854,962	3,750,202
Commercial Portfolio Segment [Member] | Mortgage Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	1,220	946
Loans and Leases Receivable, Gross	1,558,724	1,522,662	1,552,503
Commercial Portfolio Segment [Member] | Construction Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross	84,507	79,429	79,150
Commercial Portfolio Segment [Member] | Construction Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	0	0
Loans and Leases Receivable, Gross	11,461	11,151
Commercial Portfolio Segment [Member] | Construction Loans [Member] | Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	0	0
Loans and Leases Receivable, Gross	53,384	47,965
Commercial Portfolio Segment [Member] | Construction Loans [Member] | Land [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	47	67
Loans and Leases Receivable, Gross	$ 19,662	20,313
Commercial Portfolio Segment [Member] | Financing Receivables 30 To 59 Days Past Due [Member] | Mortgage Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Impaired, Description	According to the Corporation's delinquency policy and consistent with the instructions for the preparation of the Consolidated Financial Statements for Bank Holding Companies (FR Y-9C) required by the Federal Reserve Board, residential mortgage, commercial mortgage, and construction loans are considered past due when the borrower is in arrears on two or more monthly payments.
Loans and Leases Receivable, Gross	$ 6,000	5,100
Commercial Portfolio Segment [Member] | Financing Receivables 30 To 59 Days Past Due [Member] | Construction Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross	500
Commercial Portfolio Segment [Member] | Financing Receivables 30 To 59 Days Past Due [Member] | Construction Loans [Member] | Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross	$ 5,200
Commercial Portfolio Segment [Member] | Financing Receivables 30 To 59 Days Past Due [Member] | Construction Loans [Member] | Land [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Impaired, Description	According to the Corporation's delinquency policy and consistent with the instructions for the preparation of the Consolidated Financial Statements for Bank Holding Companies (FR Y-9C) required by the Federal Reserve Board, residential mortgage, commercial mortgage, and construction loans are considered past due when the borrower is in arrears on two or more monthly payments.
Loans and Leases Receivable, Gross	$ 100	200
Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross	2,015,292	1,944,713	$ 1,734,596
Consumer Portfolio Segment [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	3,750	3,752
Loans and Leases Receivable, Gross	676,409	664,701
Loans Insured or Guaranteed by US Government Authorities [Member] | Residential Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	98,996	104,751
Defaulted Loans Collateralizing Securities	44,600	43,600
Loans and Leases Receivable, Gross	$ 141,655	147,205
Loans Insured or Guaranteed by US Government Authorities [Member] | Residential Portfolio Segment [Member] | Financing Receivables 30 To 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Impaired, Description	According to the Corporation's delinquency policy and consistent with the instructions for the preparation of the Consolidated Financial Statements for Bank Holding Companies (FR Y-9C) required by the Federal Reserve Board, residential mortgage, commercial mortgage, and construction loans are considered past due when the borrower is in arrears on two or more monthly payments.
Loans and Leases Receivable, Gross	$ 5,800	5,600
Loans Insured or Guaranteed by US Government Authorities [Member] | Nonaccrual Of Interest [Member] | Residential Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days past due and still accruing	$ 47,800	$ 51,400
Period Of Residential Mortgage Loan That Are No Longer Accruing Interest	1 year 3 months