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Securities - Summary of Portfolio of Securities Available for Sale (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
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Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | $ 6,243,021 | $ 5,743,929 | [1] | $ 5,841,796 | [2]
Gross Unrealized Gain |  | 101,046 | 130,677 | [1] | 77,498 | [2]
Gross Unrealized Loss |  | 150,361 | 29,697 | [1] | 33,407 | [2]
Fair Value |  | 6,193,706 | 5,844,909 | [1] | 5,885,887 | [2]
Asset-backed Securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost | [3] |  | 530,226 |  | 384,108 |
Gross Unrealized Gain | [3] |  | 2,576 |  |  |
Gross Unrealized Loss | [3] |  | 5,703 |  | 10,854 |
Fair Value | [3] |  | 527,099 |  | 373,254 |
Mutual Funds |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 15,814 | 15,814 |  |  |
Gross Unrealized Gain |  | 116 | 269 |  |  |
Gross Unrealized Loss |  | 129 |  |  |  |
Fair Value |  | 15,801 | 16,083 |  |  |
Mutual Funds and Common Stock |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost | [4] |  |  |  | 16,871 |
Gross Unrealized Gain | [4] |  |  |  | 718 |
Gross Unrealized Loss | [4] |  |  |  | 173 |
Fair Value | [4] |  |  |  | 17,416 |
Mortgage-Related Securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 3,191,601 | 2,942,332 |  | 3,313,757 |
Gross Unrealized Gain |  | 69,310 | 99,001 |  | 47,282 |
Gross Unrealized Loss |  | 65,323 | 3,008 |  | 7,304 |
Fair Value |  | 3,195,588 | 3,038,325 |  | 3,353,735 |
Mortgage-Related Securities | GSE Certificates |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 1,183,746 | 1,155,436 |  | 1,530,317 |
Gross Unrealized Gain |  | 40,473 | 54,310 |  | 26,069 |
Gross Unrealized Loss |  | 14,901 | 136 |  | 3,763 |
Fair Value |  | 1,209,318 | 1,209,610 |  | 1,552,623 |
Mortgage-Related Securities | GSE CMOs |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 2,007,855 | 1,786,896 |  | 1,783,440 |
Gross Unrealized Gain |  | 28,837 | 44,691 |  | 21,213 |
Gross Unrealized Loss |  | 50,422 | 2,872 |  | 3,541 |
Fair Value |  | 1,986,270 | 1,828,715 |  | 1,801,112 |
Debt Securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 3,035,606 | 2,770,491 |  | 2,495,876 |
Gross Unrealized Gain |  | 31,620 | 31,206 |  | 29,376 |
Gross Unrealized Loss |  | 84,909 | 26,689 |  | 25,930 |
Fair Value |  | 2,982,317 | 2,775,008 |  | 2,499,322 |
Debt Securities | U. S. Treasury Obligations |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 64,993 | 64,984 |  | 41,820 |
Gross Unrealized Gain |  | 5 | 1 |  | 19 |
Gross Unrealized Loss |  |  |  |  | 0 |
Fair Value |  | 64,998 | 64,985 |  | 41,839 |
Debt Securities | GSE Debentures |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 1,433,280 | 1,158,253 |  | 1,093,845 |
Gross Unrealized Gain |  | 2,693 | 3,998 |  | 5,707 |
Gross Unrealized Loss |  | 67,632 | 3,949 |  | 5,312 |
Fair Value |  | 1,368,341 | 1,158,302 |  | 1,094,240 |
Debt Securities | Asset-backed Securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 520,111 | 530,226 |  |  |
Gross Unrealized Gain |  | 4,030 | 2,576 |  |  |
Gross Unrealized Loss |  | 3,368 | 5,703 |  |  |
Fair Value |  | 520,773 | 527,099 |  |  |
Debt Securities | Municipal Bonds |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 25,715 | 25,776 |  | 26,808 |
Gross Unrealized Gain |  | 482 | 625 |  | 559 |
Gross Unrealized Loss |  | 186 | 90 |  | 475 |
Fair Value |  | 26,011 | 26,311 |  | 26,892 |
Debt Securities | Corporate Bonds |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 870,886 | 870,745 |  | 854,195 |
Gross Unrealized Gain |  | 16,594 | 17,928 |  | 15,970 |
Gross Unrealized Loss |  | 5,410 | 6,447 |  | 2,983 |
Fair Value |  | 882,070 | 882,226 |  | 867,182 |
Debt Securities | Foreign Notes |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 25,000 | 25,000 |  |  |
Gross Unrealized Gain |  | 993 | 538 |  |  |
Fair Value |  | 25,993 | 25,538 |  |  |
Debt Securities | Capital Trust Notes |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 95,621 | 95,507 |  | 95,100 |
Gross Unrealized Gain |  | 6,823 | 5,540 |  | 7,121 |
Gross Unrealized Loss |  | 8,313 | 10,500 |  | 6,306 |
Fair Value |  | 94,131 | 90,547 |  | 95,915 |
Mortgage Backed Securities And Other Securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 6,227,207 | 5,712,823 |  | 5,809,633 |
Gross Unrealized Gain |  | 100,930 | 130,207 |  | 76,658 |
Gross Unrealized Loss |  | 150,232 | 29,697 |  | 33,234 |
Fair Value |  | 6,177,905 | 5,813,333 |  | 5,853,057 |
Equity Securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 15,814 | 31,106 |  | 32,163 |
Gross Unrealized Gain |  | 116 | 470 |  | 840 |
Gross Unrealized Loss |  | 129 |  |  | 173 |
Fair Value |  | $ 15,801 | 31,576 |  | 32,830 |
Equity Securities | Preferred Stock |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  |  | 15,292 |  | 15,292 |
Gross Unrealized Gain |  |  | 201 |  | 122 |
Gross Unrealized Loss |  |  |  |  | 0 |
Fair Value |  |  | $ 15,493 |  | $ 15,414 |
[1] | Excludes accrued interest receivable of $14.9 million included in other assets in the Consolidated Statements of Condition.
[2] | Excludes accrued interest receivable of $24.4 million included in other assets in the Consolidated Statements of Condition.
[3] | The underlying assets of the asset-backed securities are substantially guaranteed by the U.S. Government.
[4] | Primarily consists of mutual funds that are CRA-qualified investments.

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