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Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 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 | $ 4,187.9 |  | 4,187.9 |
Reclassification Adjustment for Amounts Included in Net Income |  |  |  |
Reclassification Adjustment for Amounts Included in Net Income | (3.5) | $ (14.6) | (7.4) | $ (27.1)
Income Tax Benefit (Expense) | 0.8 | 3.1 | 1.6 | 5.7
Net of Taxes | (2.7) | (11.5) | (5.8) | (21.4)
Changes in Net Unrecognized Postretirement Benefit Costs |  |  |  |
Net Unrecognized Postretirement Benefit Costs | 0.7 | 0.0 | 1.4 | 0.6
Income Tax Benefit (Expense) | (0.1) | 0.1 | (0.3) | (0.1)
Net of Taxes | 0.6 | 0.1 | 1.1 | 0.5
Changes in Gain (Loss) on Cash Flow Hedges |  |  |  |
Gain on Cash Flow Hedges | 0.0 | 0.0 | 0.1 | 0.1
Income Tax Benefit (Expense) | 0.0 | 0.0 | 0.0 | 0.0
Net of Taxes | 0.0 | 0.0 | 0.1 | 0.1
Total Other Comprehensive Income (Loss) | 408.0 | 159.3 | 204.0 | 322.8
Total Income Tax Benefit (Expense) | (85.6) | (33.3) | (42.8) | (67.7)
Other Comprehensive Income (Loss) | 322.4 | 126.0 | 161.2 | 255.1
No Credit Losses Recognized in Consolidated Statements of Income |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities: |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities | 411.6 | 173.9 | 213.6 | 349.2
Income Tax Benefit (Expense) | (86.4) | (36.5) | (44.9) | (73.3)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax | 325.2 | 137.4 | 168.7 | 275.9
Credit Losses Recognized in Consolidated Statements of Income |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities: |  |  |  |
Changes in Net Unrealized Gains (Losses) on Investment Securities | (0.8) | 0.0 | (3.7) | 0.0
Income Tax Benefit (Expense) | 0.2 | 0.0 | 0.8 | 0.0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax | (0.6) | 0.0 | (2.9) | 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 | 279.9 |  | 439.4 |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 325.2 |  | 168.7 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | (2.8) |  | (5.8) |
Other Comprehensive Income (Loss), Net of Tax | 322.4 |  | 162.9 |
Balance at End of Period | 602.3 |  | 602.3 |
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 | (2.3) |  | 0.0 |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | (0.6) |  | (2.9) |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | 0.0 |  | 0.0 |
Other Comprehensive Income (Loss), Net of Tax | (0.6) |  | (2.9) |
Balance at End of Period | (2.9) |  | (2.9) |
Net Unrecognized Postretirement Benefit Costs |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period | (100.1) |  | (100.6) |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 0.6 |  | 1.1 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | 0.0 |  | 0.0 |
Other Comprehensive Income (Loss), Net of Tax | 0.6 |  | 1.1 |
Balance at End of Period | (99.5) |  | (99.5) |
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 | 0.0 |  | 0.1 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | 0.0 |  | 0.0 |
Other Comprehensive Income (Loss), Net of Tax | 0.0 |  | 0.1 |
Balance at End of Period | (2.6) |  | (2.6) |
Accumulated Other Comprehensive Income |  |  |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items] |  |  |  |
Balance at Beginning of Period | 174.9 |  | 336.1 |
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax | 325.2 |  | 167.0 |
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax | (2.8) |  | (5.8) |
Other Comprehensive Income (Loss), Net of Tax | 322.4 |  | 161.2 |
Balance at End of Period | 497.3 |  | 497.3 |
Changes in Gain (Loss) on Cash Flow Hedges |  |  |  |
Other Comprehensive Income (Loss) | $ 322.4 | $ 126.0 | $ 161.2 | $ 255.1

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