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Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Investment securities available for sale |  |
Investment in marketable equity securities at cost | $ 85,554 | $ 84,837
Investment in marketable equity securities, gross unrealized gains | 37,744 | 8,654
Investment in marketable equity securities, gross unrealized losses | 151 | 1,811
Fair value | 123,147 | 91,680
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 3,381,078 | 2,816,982
Gross unrealized gains | 5,316 |
Gross unrealized losses | 33,316 |
Fair value | 3,353,078 | 2,838,499
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Cost | 10,811,775 | 9,813,784
Gross unrealized gains | 102,797 | 141,416
Gross unrealized losses | 67,218 | 10,778
Fair value | 10,847,354 | 9,944,422
Investment securities available for sale at cost | 7,345,143 | 6,911,965
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 59,737 | 111,189
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 33,751 | 8,911
Debt Securities, Available-for-sale | 7,371,129 | 7,014,243
Interest receivable, available-for-sale debt securities | 21,100 | 17,600
Interest receivable, held-to-maturity debt securities | 6,000 | 5,400
U.S. Treasury |  |
Investment securities held to maturity |  |
Investment securities available for sale at cost |  | 499,832
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax |  | 101
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax |  | 0
Debt Securities, Available-for-sale |  | 499,933
Government agency |  |
Investment securities held to maturity |  |
Investment securities available for sale at cost | 851,860 | 706,241
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 5,348 | 723
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 3,480 | 5,573
Debt Securities, Available-for-sale | 853,728 | 701,391
Residential Mortgage Backed Securities |  |
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 2,122,299 | 1,877,692
Gross unrealized gains | 5,313 | 17,689
Gross unrealized losses | 15,051 | 0
Fair value | 2,112,561 | 1,895,381
Investment securities available for sale at cost | 4,800,194 | 4,369,130
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 19,219 | 70,283
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 17,767 | 1,310
Debt Securities, Available-for-sale | 4,801,646 | 4,438,103
Commercial Mortgage Backed Securities |  |
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 1,256,771 | 937,034
Gross unrealized gains | 3 | 3,884
Gross unrealized losses | 18,265 | 56
Fair value | 1,238,509 | 940,862
Investment securities available for sale at cost | 1,092,183 | 745,892
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 9,875 | 25,645
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 11,406 | 0
Debt Securities, Available-for-sale | 1,090,652 | 771,537
Residential mortgage-backed securities |  |
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total |  | 2,816,982
Gross unrealized gains |  | 21,573
Gross unrealized losses |  | 56
Fair value |  | 2,838,499
Commercial mortgage-backed securities |  |
Investment securities held to maturity |  |
Investment securities available for sale at cost | 600,906 | 590,870
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 25,295 | 14,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 1,098 | 2,028
Debt Securities, Available-for-sale | 625,103 | 603,279
Other |  |
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 2,008 | 2,256
Gross unrealized gains | 0 | 0
Gross unrealized losses | 0 | 0
Fair value | $ 2,008 | $ 2,256

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