Loans and Leases Receivable and Allowance for Credit Losses - Additional Information (Detail)			3 Months Ended			6 Months Ended
	Jun. 30, 2020 USD ($) Commitment	Jan. 01, 2020 USD ($)	Jun. 30, 2020 USD ($) Commitment Loan	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($) Loan	Jun. 30, 2020 USD ($) Commitment Loan	Jun. 30, 2019 USD ($) Loan	Dec. 31, 2019 USD ($) Commitment	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Financing Receivable, Troubled Debt Restructuring [Line Items]
Accrued interest receivable	$ 49,911,000		$ 49,911,000			$ 49,911,000		$ 38,072,000
Loans receivable	14,666,133,000		14,666,133,000			14,666,133,000		9,508,367,000
Interest income			118,447,000		$ 103,567,000	$ 234,527,000	$ 196,683,000
Loans held for sale, average life from purchase to sale						30 days
Interest income, nonaccrual loans			600,000		300,000	$ 700,000	500,000
Allowance for credit losses on loans and leases	159,905,000	$ 136,200,000	159,905,000	$ 149,283,000	$ 48,388,000	159,905,000	$ 48,388,000	56,379,000	$ 43,679,000	$ 39,972,000
Allowance for credit loss, period increase						23,700,000
Loans reported as TDR	14,800,000		14,800,000			$ 14,800,000		13,300,000
Minimum performance requirement						9 months
Lease receivable, TDR	$ 0		$ 0			$ 0		$ 0
Number of loans | Loan			22		2	38	14
Number of commitments to lend additional funds (commitment) | Commitment	0		0			0		0
Purchased-credit-impaired loans	$ 15,700,000			17,488,000
Loans pledged as collateral	8,500,000,000		$ 8,500,000,000			$ 8,500,000,000		$ 4,600,000,000
Small Business Administration (SBA), CARES Act, Paycheck Protection Program
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans receivable	4,800,000,000		4,800,000,000			4,800,000,000
Interest income			11,700,000			11,700,000
Accounting Standards Update 2016-13
Financing Receivable, Troubled Debt Restructuring [Line Items]
Initial allowance on loans purchased with credit deterioration		200,000
Financing receivable, purchased with credit deterioration, discount		$ 300,000
Residential real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Residential real estate held in other real estate owned	100,000		100,000			100,000		200,000
Loans in process of foreclosure	700,000		700,000			700,000		900,000
Commercial real estate non-owner occupied
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amount deferred, CARES Act	974,000,000.0		974,000,000.0			974,000,000.0
Multi-family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Allowance for credit losses on loans and leases	14,697,000		14,697,000	8,750,000	$ 9,926,000	14,697,000	$ 9,926,000	6,157,000	$ 10,630,000	$ 11,462,000
Purchased-credit-impaired loans				$ 1,688,000
Consumer loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Amount deferred, CARES Act	81,100,000		$ 81,100,000			$ 81,100,000
Forgiveness of debt
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of loans | Loan			0		0	0	0
Other Assets
Financing Receivable, Troubled Debt Restructuring [Line Items]
Accrued interest receivable	47,100,000		$ 47,100,000			$ 47,100,000		$ 34,800,000
Commercial Real Estate | Commercial and industrial | Collateral Dependent Loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans receivable	55,900,000		55,900,000			55,900,000
Government-Guaranteed Collateral
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans pledged as collateral	$ 4,400,000,000		$ 4,400,000,000			$ 4,400,000,000