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Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Investment securities available for sale |  |
Available-for-sale Securities, Amortized Cost Basis, Total | $ 7,335,745 | $ 6,911,965
Gross unrealized losses | 26,484 | 8,911
Fair value | 7,381,083 | 7,014,243
Investment in marketable equity securities at cost | 84,297 | 84,837
Investment in marketable equity securities, gross unrealized gains | 34,415 | 8,654
Investment in marketable equity securities, gross unrealized losses | 172 | 1,811
Fair value | 118,540 | 91,680
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 3,394,604 | 2,816,982
Gross unrealized gains | 7,092 |
Gross unrealized losses | 24,611 |
Fair value | 3,377,085 | 2,838,499
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Cost | 10,814,646 | 9,813,784
Gross unrealized gains | 113,329 | 141,416
Gross unrealized losses | 51,267 | 10,778
Fair value | 10,876,708 | 9,944,422
Investment securities available for sale at cost | 7,335,745 | 6,911,965
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 71,822 | 111,189
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 26,484 | 8,911
Debt Securities, Available-for-sale | 7,381,083 | 7,014,243
Interest receivable, available-for-sale debt securities | 18,000 | 17,600
Interest receivable, held-to-maturity debt securities | 6,100 | 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 available for sale |  |
Gross unrealized losses | 3,747 | 5,573
Investment securities held to maturity |  |
Investment securities available for sale at cost | 824,216 | 706,241
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 4,981 | 723
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 3,747 | 5,573
Debt Securities, Available-for-sale | 825,450 | 701,391
Residential Mortgage Backed Securities |  |
Investment securities available for sale |  |
Gross unrealized losses | 14,324 | 1,310
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 2,208,496 | 1,877,692
Gross unrealized gains | 6,567 | 17,689
Gross unrealized losses | 12,189 | 0
Fair value | 2,202,874 | 1,895,381
Investment securities available for sale at cost | 4,770,205 | 4,369,130
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 29,375 | 70,283
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 14,324 | 1,310
Debt Securities, Available-for-sale | 4,785,256 | 4,438,103
Commercial Mortgage Backed Securities |  |
Investment securities available for sale |  |
Available-for-sale Securities, Amortized Cost Basis, Total | 1,144,024 | 745,892
Gross unrealized gains | 12,702 | 25,645
Gross unrealized losses | 8,196 | 0
Fair value | 1,148,530 | 771,537
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 1,184,101 | 937,034
Gross unrealized gains | 525 | 3,884
Gross unrealized losses | 12,422 | 56
Fair value | 1,172,204 | 940,862
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 available for sale |  |
Gross unrealized losses | 217 |
Investment securities held to maturity |  |
Investment securities available for sale at cost | 597,300 | 590,870
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 24,764 | 14,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 217 | 2,028
Debt Securities, Available-for-sale | 621,847 | 603,279
Other |  |
Investment securities held to maturity |  |
Debt Securities, Held-to-maturity, Total | 2,007 | 2,256
Gross unrealized gains | 0 | 0
Gross unrealized losses | 0 | 0
Fair value | $ 2,007 | $ 2,256

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