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Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period |  |  | $ 3,972.3 |
Balance at End of Period | $ 4,347.5 |  | 4,347.5 |
Reclassification Adjustment for Amounts Included in Net Income |  |  |  |
Reclassification Adjustment for Amounts Included in Net Income | (9.0) | $ 0.2 | (16.4) | $ (26.9)
Income Tax Benefit (Expense) | 1.9 | 0.0 | 3.4 | 5.7
Net of Taxes | (7.1) | 0.2 | (13.0) | (21.2)
Changes in Net Unrecognized Postretirement Benefit Costs |  |  |  |
Net Unrecognized Postretirement Benefit Costs | 0.9 | (0.4) | 2.3 | 0.2
Income Tax Benefit (Expense) | (0.2) | 0.0 | (0.5) | (0.1)
Net of Taxes | 0.7 | (0.4) | 1.8 | 0.1
Changes in Gain (Loss) on Cash Flow Hedges |  |  |  |
Gain on Cash Flow Hedges | 0.2 | 0.2 | 0.3 | 0.3
Income Tax Benefit (Expense) | (0.1) | (0.1) | (0.1) | (0.1)
Net of Taxes | 0.1 | 0.1 | 0.2 | 0.2
Total Other Comprehensive Income (Loss) Before Income Taxes | 65.5 | 118.1 | 269.5 | 440.9
Total Income Tax Benefit (Expense) | (13.8) | (24.9) | (56.6) | (92.6)
Other Comprehensive Income (Loss), Net of Taxes | 51.7 | 93.2 | 212.9 | 348.3
No Credit Losses Recognized in Condensed Consolidated Statements of Income |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities: |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities | 72.1 | 118.1 | 285.7 | 467.3
Income Tax Benefit (Expense) | (15.1) | (24.8) | (59.9) | (98.1)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax | 57.0 | 93.3 | 225.8 | 369.2
Credit Losses Recognized in Condensed Consolidated Statements of Income |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities: |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities | 1.3 | 0.0 | (2.4) | 0.0
Income Tax Benefit (Expense) | (0.3) | 0.0 | 0.5 | 0.0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax | 1.0 | 0.0 | (1.9) | 0.0
Having No Credit Losses Recognized in Consolidated Statements of Income |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period | 602.3 |  | 439.4 |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 57.0 |  | 225.8 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | (7.1) |  | (13.0) |
Other Comprehensive Income (Loss), Net of Tax | 49.9 |  | 212.8 |
Balance at End of Period | 652.2 |  | 652.2 |
Having Credit Losses Recognized in Consolidated Statements of Income |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period | (2.9) |  | 0.0 |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 1.0 |  | (1.9) |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | 0.0 |  | 0.0 |
Other Comprehensive Income (Loss), Net of Tax | 1.0 |  | (1.9) |
Balance at End of Period | (1.9) |  | (1.9) |
Net Unrecognized Postretirement Benefit Costs |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period | (99.5) |  | (100.6) |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 0.7 |  | 1.8 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | 0.0 |  | 0.0 |
Other Comprehensive Income (Loss), Net of Tax | 0.7 |  | 1.8 |
Balance at End of Period | (98.8) |  | (98.8) |
Gain (Loss) on Cash Flow Hedges |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period | (2.6) |  | (2.7) |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 0.1 |  | 0.2 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | 0.0 |  | 0.0 |
Other Comprehensive Income (Loss), Net of Tax | 0.1 |  | 0.2 |
Balance at End of Period | (2.5) |  | (2.5) |
Accumulated Other Comprehensive Income |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period | 497.3 |  | 336.1 |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 58.8 |  | 225.9 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | (7.1) |  | (13.0) |
Other Comprehensive Income (Loss), Net of Tax | 51.7 |  | 212.9 |
Balance at End of Period | 549.0 |  | 549.0 |
Changes in Gain (Loss) on Cash Flow Hedges |  |  |  |
Other Comprehensive Income (Loss), Net of Taxes | $ 51.7 | $ 93.2 | $ 212.9 | $ 348.3

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