Allowance for Loan Losses, Allowance for Losses on Lending-Related Commitments and Impaired Loans (Summary of the Post-Modification Balance of TDRs) (Detail) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) contracts	Mar. 31, 2018 USD ($) contracts	Mar. 31, 2019 USD ($) Loan	Mar. 31, 2018 USD ($) Loan
Financing Receivable, Modifications [Line Items]
TDRs, number	30	7	94	24
TDRs, Additions	$ 23,718	$ 990	$ 51,969	$ 23,802
Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	22	7
TDRs, Additions	$ 4,994	$ 990
Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	6	2
TDRs, Additions	$ 1,547	$ 111
Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	8	0
TDRs, Additions	$ 18,724	$ 0
Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Residential real estate and other
Financing Receivable, Modifications [Line Items]
TDRs, number	20	5	74	15
TDRs, Additions	$ 4,486	$ 835	$ 13,411	$ 3,711
Residential real estate and other | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	20	5
TDRs, Additions	$ 4,486	$ 835
Residential real estate and other | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	6	2
TDRs, Additions	$ 1,547	$ 111
Residential real estate and other | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Residential real estate and other | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Commercial, industrial and other | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	8	1	11	5
TDRs, Additions	$ 18,854	$ 96	$ 32,199	$ 3,776
Commercial, industrial and other | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	1
TDRs, Additions	$ 432	$ 96
Commercial, industrial and other | Commercial | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Commercial, industrial and other | Commercial | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	7	0
TDRs, Additions	$ 18,422	$ 0
Commercial, industrial and other | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Asset-based lending | Commercial
Financing Receivable, Modifications [Line Items]
TDRs, number	1		2	0
TDRs, Additions	$ 76		$ 206	$ 0
Asset-based lending | Commercial | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1
TDRs, Additions	$ 76
Asset-based lending | Commercial | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0
TDRs, Additions	$ 0
Asset-based lending | Commercial | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0
TDRs, Additions	$ 0
Asset-based lending | Commercial | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0
TDRs, Additions	$ 0
Office | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number		1	0	1
TDRs, Additions		$ 59	$ 0	$ 59
Office | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts		1
TDRs, Additions		$ 59
Office | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts		0
TDRs, Additions		$ 0
Office | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts		0
TDRs, Additions		$ 0
Office | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts		0
TDRs, Additions		$ 0
Mixed use and other | Commercial real estate
Financing Receivable, Modifications [Line Items]
TDRs, number	1	0	3	0
TDRs, Additions	$ 302	$ 0	$ 757	$ 0
Mixed use and other | Commercial real estate | Extension at Below Market Terms
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Mixed use and other | Commercial real estate | Reduction of Interest Rate
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0
Mixed use and other | Commercial real estate | Modification to Interest Only Payments
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	1	0
TDRs, Additions	$ 302	$ 0
Mixed use and other | Commercial real estate | Forgiveness of Debt
Financing Receivable, Modifications [Line Items]
TDRs, number | contracts	0	0
TDRs, Additions	$ 0	$ 0