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Investments - Narrative (Details) - USD ($) $ in Millions | Mar. 31, 2022 | Dec. 31, 2021
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Investment Holdings [Line Items] |  |
Investment grade rating | 90.60% |
Debt securities, available-for-sale | $ 5,987.4 | $ 6,239.3
FHLB funding agreements | 847.5 | 787.5
FHLB of Chicago |  |
Investment Holdings [Line Items] |  |
Debt securities, available-for-sale | 921.0 | 870.1
Governmental Agencies as Required by Law in Various States |  |
Investment Holdings [Line Items] |  |
Debt securities, available-for-sale | $ 29.1 | $ 26.2

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