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Debt Securities - Credit Quality Indictors for HTM Debt Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items] |  |
Debt securities, held-to-maturity | $ 1,128,299 | $ 189,836
Investment Grade | Moody's: Aaa | S&P: AAA |  |
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items] |  |
Debt securities, held-to-maturity | 274,791 | 39,022
Investment Grade | Moody's: Aa1, Aa2, Aa3 | S&P: AA+, AA, AA- |  |
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items] |  |
Debt securities, held-to-maturity | 812,759 | 123,664
Investment Grade | Moody's: A1, A2, A3 | S&P: A+, A, A- |  |
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items] |  |
Debt securities, held-to-maturity | 38,739 | 27,150
Investment Grade | Moody’s: Baa1, Baa2, Baa3 | S&P's, BBB+, BBB, BBB- |  |
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items] |  |
Debt securities, held-to-maturity | 0 | 0
Investment Grade | Moody's: Not Rated | S&P: Not Rated |  |
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items] |  |
Debt securities, held-to-maturity | 2,010 | 0
Below investment grade |  |
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items] |  |
Debt securities, held-to-maturity | $ 0 | $ 0

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