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Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Investment securities available for sale |  |
Investment securities available for sale, cost | $ 6,911,965 | $ 7,052,152
Investment securities available for sale, gross unrealized gains | 111,189 | 25,341
Investment securities available for sale, gross unrealized losses | 8,911 | 17,819
Allowance for credit losses | 0 |
Investment securities available for sale, fair value | 7,014,243 | 7,059,674
Investment in marketable equity securities, cost | 84,837 | 59,262
Investment in marketable equity securities, gross unrealized gains | 8,654 | 23,304
Investment in marketable equity securities, gross unrealized losses | 1,811 | 233
Investment in marketable equity securities, fair value | 91,680 | 82,333
Investment securities held to maturity |  |
Investment securities held to maturity, cost | 2,816,982 | 30,996
Investment securities held to maturity, gross unrealied gains | 21,573 |
Investment securities held to maturity, gross unrealized losses | 56 |
Debt securities, held-to-maturity, allowance for credit loss | 0 |
Investment securities held to maturity, fair value | 2,838,499 | 30,996
Total investment securities, cost | 9,813,784 | 7,142,410
Total investment securities, gross unrealized gains | 141,416 | 48,645
Total investment securities, fair value | 10,778 | 18,052
Debt Securities, Available-for-sale, Held-to-maturity and Equity Securities, Allowance for Credit Losses | 0 |
Total investment securities, gross unrealized losses | 9,944,422 | 7,173,003
U. S. Treasury |  |
Investment securities available for sale |  |
Investment securities available for sale, cost | 499,832 | 409,397
Investment securities available for sale, gross unrealized gains | 101 | 602
Investment securities available for sale, gross unrealized losses | 0 | 0
Allowance for credit losses | 0 |
Investment securities available for sale, fair value | 499,933 | 409,999
Government Agency |  |
Investment securities available for sale |  |
Investment securities available for sale, cost | 706,241 | 684,085
Investment securities available for sale, gross unrealized gains | 723 | 928
Investment securities available for sale, gross unrealized losses | 5,573 | 2,241
Allowance for credit losses | 0 |
Investment securities available for sale, fair value | 701,391 | 682,772
Residential Mortgage Backed Securities |  |
Investment securities available for sale |  |
Investment securities available for sale, cost | 4,369,130 | 5,269,060
Investment securities available for sale, gross unrealized gains | 70,283 | 13,417
Investment securities available for sale, gross unrealized losses | 1,310 | 15,387
Allowance for credit losses | 0 |
Investment securities available for sale, fair value | 4,438,103 | 5,267,090
Investment securities held to maturity |  |
Investment securities held to maturity, cost | 1,877,692 | 0
Investment securities held to maturity, gross unrealied gains | 17,689 |
Investment securities held to maturity, gross unrealized losses | 0 |
Debt securities, held-to-maturity, allowance for credit loss | 0 |
Investment securities held to maturity, fair value | 1,895,381 |
Commercial Mortgage Backed Securities |  |
Investment securities available for sale |  |
Investment securities available for sale, cost | 745,892 | 373,105
Investment securities available for sale, gross unrealized gains | 25,645 | 6,974
Investment securities available for sale, gross unrealized losses | 0 | 59
Allowance for credit losses | 0 |
Investment securities available for sale, fair value | 771,537 | 380,020
Investment securities held to maturity |  |
Investment securities held to maturity, cost | 937,034 | 0
Investment securities held to maturity, gross unrealied gains | 3,884 |
Investment securities held to maturity, gross unrealized losses | 56 |
Debt securities, held-to-maturity, allowance for credit loss | 0 |
Investment securities held to maturity, fair value | 940,862 |
Corporate Bonds |  |
Investment securities available for sale |  |
Investment securities available for sale, cost | 590,870 | 198,278
Investment securities available for sale, gross unrealized gains | 14,437 | 3,420
Investment securities available for sale, gross unrealized losses | 2,028 | 132
Allowance for credit losses | 0 |
Investment securities available for sale, fair value | 603,279 | 201,566
US States and Political Subdivisions Debt Securities |  |
Investment securities available for sale |  |
Investment securities available for sale, cost |  | 118,227
Investment securities available for sale, gross unrealized gains |  | 0
Investment securities available for sale, gross unrealized losses |  | 0
Investment securities available for sale, fair value |  | 118,227
Other Debt Obligations |  |
Investment securities held to maturity |  |
Investment securities held to maturity, cost | 2,256 | 30,996
Investment securities held to maturity, gross unrealied gains | 0 | 0
Investment securities held to maturity, gross unrealized losses | 0 | 0
Debt securities, held-to-maturity, allowance for credit loss | 0 |
Investment securities held to maturity, fair value | $ 2,256 | $ 30,996

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