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Investments - Narrative (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
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Investment Holdings [Line Items] |  |
Investment grade rating | 91.80% |
Debt securities, available-for-sale | $ 6,356.3 | $ 6,345.3
FHLB funding agreements | 774.5 | 644.5
FHLB of Chicago |  |
Investment Holdings [Line Items] |  |
Debt securities, available-for-sale | 844.1 | 707.3
Governmental Agencies as Required by Law in Various States |  |
Investment Holdings [Line Items] |  |
Debt securities, available-for-sale | $ 26.6 | $ 26.9

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