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Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Investment securities available for sale |  |
Amortized cost | $ 7,270,552 | $ 6,911,965
Gross unrealized losses | 44,888 | 8,911
Fair value | 7,307,122 | 7,014,243
Investment in marketable equity securities at cost | 84,297 | 84,837
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax | 22,872 | 8,654
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax | 283 | 1,811
Fair value | 106,886 | 91,680
Investment securities held to maturity |  |
Amortized cost | 2,808,099 | 2,816,982
Gross unrealized gains | 5,204 |
Gross unrealized losses | 38,321 |
Fair value | 2,774,982 | 2,838,499
Amortized cost | 10,162,948 | 9,813,784
Gross unrealized gains | 109,534 | 141,416
Gross unrealized losses | 83,492 | 10,778
Fair value | 10,188,990 | 9,944,422
Investment securities available for sale at cost | 7,270,552 | 6,911,965
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 81,458 | 111,189
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 44,888 | 8,911
Debt Securities, Available-for-sale | 7,307,122 | 7,014,243
Interest receivable, available-for-sale debt securities | 21,300 | 17,600
Interest receivable, held-to-maturity debt securities | 5,300 | 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 | 6,403 | 5,573
Investment securities held to maturity |  |
Investment securities available for sale at cost | 852,305 | 706,241
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 784 | 723
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 6,403 | 5,573
Debt Securities, Available-for-sale | 846,686 | 701,391
Residential Mortgage Backed Securities |  |
Investment securities available for sale |  |
Gross unrealized losses | 26,046 | 1,310
Investment securities held to maturity |  |
Amortized cost | 1,740,314 | 1,877,692
Gross unrealized gains | 5,191 | 17,689
Gross unrealized losses | 20,510 | 0
Fair value | 1,724,995 | 1,895,381
Investment securities available for sale at cost | 4,736,893 | 4,369,130
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 47,742 | 70,283
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 26,046 | 1,310
Debt Securities, Available-for-sale | 4,758,589 | 4,438,103
Commercial Mortgage Backed Securities |  |
Investment securities available for sale |  |
Amortized cost | 1,096,259 | 745,892
Gross unrealized gains | 14,619 | 25,645
Gross unrealized losses | 11,891 | 0
Fair value | 1,098,987 | 771,537
Investment securities held to maturity |  |
Amortized cost | 1,065,778 | 937,034
Gross unrealized gains | 13 | 3,884
Gross unrealized losses | 17,811 | 56
Fair value | 1,047,980 | 940,862
Residential mortgage-backed securities |  |
Investment securities held to maturity |  |
Amortized cost |  | 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 | 548 |
Investment securities held to maturity |  |
Investment securities available for sale at cost | 585,095 | 590,870
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 18,313 | 14,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 548 | 2,028
Debt Securities, Available-for-sale | 602,860 | 603,279
Other |  |
Investment securities held to maturity |  |
Amortized cost | 2,007 | 2,256
Gross unrealized gains | 0 | 0
Gross unrealized losses | 0 | 0
Fair value | $ 2,007 | $ 2,256

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