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Summary of Significant Accounting Policies - Additional Information (Detail) |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jan. 01, 2020 USD ($) | Mar. 31, 2021 USD ($) shares | Mar. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) Segment shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Cash and cash equivalents, interest-bearing deposits in other financial institutions |  | $ 2,722,915,000 |  | $ 1,947,931,000 | $ 741,870,000 | $ 1,474,955,000 | $ 2,528,169,000
Cash and cash equivalents, interest-bearing deposits in other financial institutions, Federal Reserve Bank of New York |  |  |  | 1,600,000,000 | 608,400,000 |  |
Cash and cash equivalents, federal funds sold |  |  |  | 0 | 1,700,000 |  |
Cash and cash equivalents, outstanding reverse repurchase agreements |  |  |  | 193,500,000 | 0 |  |
Allowance for loan and lease losses |  | 197,758,000 | $ 162,244,000 | 194,043,000 | 147,638,000 | 159,820,000 |
Retained earnings |  | 552,566,000 |  | 494,229,000 | 342,023,000 |  |
Increase in allowance for credit losses on loans and leases | $ 1,900,000 | 4,841,000 | 189,000 | 3,393,000 | 1,020,000 |  |
Off-balance sheet credit exposures before adoption of CECL | 12,500,000 |  |  |  |  |  |
Goodwill |  | 2,426,379,000 |  | $ 2,426,379,000 | 2,426,379,000 |  |
Number of operating segments | Segment |  |  |  | 1 |  |  |
Number of reportable segments | Segment |  |  |  | 1 |  |  |
Goodwill impairment |  |  |  | $ 0 |  |  |
Depreciation |  | 5,357,000 | 6,407,000 | 23,871,000 | 27,096,000 | 32,323,000 |
Bank-owned life insurance |  | 1,165,765,000 |  | 1,164,196,000 | 1,145,058,000 |  |
Additional purchases of BOLI |  |  |  |  | 150,000,000.0 |  |
Bank-owned life insurance income |  | $ 6,890,000 | $ 7,389,000 | 31,750,000 | 28,363,000 | $ 28,252,000 |
Other real estate owned |  |  |  | $ 1,800,000 | 2,000,000.0 |  |
Shares available for grant | shares |  | 8,481,554 |  | 11,913,461 |  |  |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Allowance for loan and lease losses |  |  |  |  | 1,911,000 |  |
Taxi Medallion Loans |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Other real estate owned |  |  |  | $ 6,500,000 | 10,300,000 |  |
Premises |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) |  |  |  | 20 years |  |  |
Furniture, Fixtures and Equipment | Minimum |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) |  |  |  | 3 years |  |  |
Furniture, Fixtures and Equipment | Maximum |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Premises, furniture, fixtures, and equipment, estimated useful lives (in years) |  |  |  | 10 years |  |  |
Unfunded Commitments |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Allowance for credit losses |  | $ 12,300,000 |  | $ 11,900,000 | $ 461,000 |  |
Accounting Standards Update 2016-13 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Retained earnings | $ (1,900,000) |  |  |  |  |  |
Change in accounting principle, accounting standards update, adopted [true false] |  |  |  | true |  |  |
Change in accounting principle, accounting standards update, adoption date |  |  |  | Jan. 01, 2020 |  |  |
Accounting Standards Update 2016-13 | Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Retained earnings |  |  |  | $ (10,500,000) |  |  |
Accounting Standards Update 2020-04 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Change in accounting principle, accounting standards update, adopted [true false] |  |  |  | true |  |  |
Change in accounting principle, accounting standards update, adoption date |  |  |  | Mar. 31, 2020 |  |  |
Change in accounting principle, accounting standards update, immaterial effect [true false] |  |  |  | true |  |  |
Accounting Standards Update 2018-13 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Change in accounting principle, accounting standards update, adopted [true false] |  |  |  | true |  |  |
Change in accounting principle, accounting standards update, adoption date |  |  |  | Jan. 01, 2020 |  |  |
Change in accounting principle, accounting standards update, immaterial effect [true false] |  |  |  | true |  |  |
Accounting Standards Update 2017-04 |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Change in accounting principle, accounting standards update, adopted [true false] |  |  |  | true |  |  |
Change in accounting principle, accounting standards update, adoption date |  |  |  | Jan. 01, 2020 |  |  |
Change in accounting principle, accounting standards update, immaterial effect [true false] |  |  |  | true |  |  |

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