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Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Investment securities available for sale |  |
Cost | $ 4,898,771 | $ 4,607,117
Gross unrealized gains | 15,594 | 6,425
Gross unrealized losses | 9,482 | 56,432
Fair value | 4,904,883 | 4,557,110
Cost | 87,588 | 73,809
Gross unrealized gains | 29,605 | 19,010
Gross unrealized losses | 339 | 220
Fair value | 116,854 | 92,599
Investment securities held to maturity |  |
Cost | 2,145,943 | 2,184,653
Gross unrealized gains | 71,857 |
Gross unrealized losses | 0 |
Fair value | 2,217,800 | 2,201,502
Cost | 7,132,302 | 6,865,579
Gross unrealized gains | 117,056 | 42,774
Gross unrealized losses | 9,821 | 57,142
Fair value | 7,239,537 | 6,851,211
U.S. Treasury |  |
Investment securities available for sale |  |
Cost | 748,206 | 1,249,243
Gross unrealized gains | 1,114 | 633
Gross unrealized losses | 2 | 2,166
Fair value | 749,318 | 1,247,710
Government agency |  |
Investment securities available for sale |  |
Cost | 666,029 | 257,252
Gross unrealized gains | 1,404 | 222
Gross unrealized losses | 1,812 | 639
Fair value | 665,621 | 256,835
Mortgage-backed securities |  |
Investment securities available for sale |  |
Cost | 3,329,593 | 2,956,793
Gross unrealized gains | 10,369 | 5,309
Gross unrealized losses | 7,343 | 52,763
Fair value | 3,332,619 | 2,909,339
Investment securities held to maturity |  |
Cost | 2,115,141 | 2,184,653
Gross unrealized gains | 71,857 | 17,339
Gross unrealized losses | 0 | 490
Fair value | 2,186,998 | 2,201,502
Corporate bonds |  |
Investment securities available for sale |  |
Cost | 154,943 | 143,829
Gross unrealized gains | 2,707 | 261
Gross unrealized losses | 325 | 864
Fair value | 157,325 | $ 143,226
Other |  |
Investment securities held to maturity |  |
Cost | 30,802 |
Gross unrealized gains | 0 |
Gross unrealized losses | 0 |
Fair value | $ 30,802 |

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