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Stockholders' Equity - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss), pre-tax | $ 1,949.6 | $ (6,249.3) | $ 986.6
Other comprehensive income (loss), tax | (415.8) | 1,340.5 | (329.3)
Other comprehensive income (loss) | 1,533.8 | (4,908.8) | 657.3
Net unrealized gains (losses) on available-for-sale securities including NCI |  |  |
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss) before reclassifications, pre-tax | 1,986.8 | (13,158.9) | (2,984.3)
Adjustments for assumed changes in amortization patterns, pre-tax | (2.6) | (3.7) | (0.1)
Adjustments for assumed changes in policyholder liabilities, pre-tax | 0.7 | 273.2 | 266.9
Other comprehensive income (loss), pre-tax | 2,336.0 | (12,490.5) | (2,698.1)
Other comprehensive income (loss) before reclassifications, tax | (419.8) | 2,787.9 | 675.1
Reclassification from accumulated other comprehensive income, tax | (73.5) | (87.6) | (5.1)
Adjustments for assumed changes in amortization patterns, tax | 0.6 | 0.7 |
Adjustments for assumed changes in policyholder liabilities, tax | (0.2) | (57.3) | (57.2)
Other comprehensive income (loss), tax | (492.9) | 2,643.7 | 612.8
Other comprehensive income (loss) before reclassifications, after-tax | 1,567.0 | (10,371.0) | (2,309.2)
Reclassification from accumulated other comprehensive income, after-tax | 277.6 | 311.3 | 14.3
Adjustments for assumed changes in amortization patterns, after-tax | (2.0) | (3.0) | (0.1)
Adjustments for assumed changes in policyholder liabilities, after-tax | 0.5 | 215.9 | 209.7
Other comprehensive income (loss) | 1,843.1 | (9,846.8) | (2,085.3)
Net unrealized gains (losses) on available-for-sale securities including NCI | Net realized capital gains (losses) |  |  |
Other Comprehensive Income (Loss) |  |  |
Reclassification from accumulated other comprehensive income, pre-tax | 351.1 | 398.9 | 19.4
Net unrealized gains (losses) on derivative instruments including NCI |  |  |
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss) before reclassifications, pre-tax | (44.9) | (1.4) | 66.7
Reclassification from accumulated other comprehensive income, pre-tax | (7.4) | (28.0) | (25.5)
Adjustments for assumed changes in amortization patterns, pre-tax | (0.9) | (0.1) |
Adjustments for assumed changes in policyholder liabilities, pre-tax | 0.2 | 0.4 | 1.2
Other comprehensive income (loss), pre-tax | (53.0) | (29.1) | 42.4
Other comprehensive income (loss) before reclassifications, tax | 9.4 | 0.3 | (14.0)
Reclassification from accumulated other comprehensive income, tax | 1.6 | 5.8 | 5.4
Adjustments for assumed changes in amortization patterns, tax | 0.2 |  |
Adjustments for assumed changes in policyholder liabilities, tax |  | (0.1) | (0.4)
Other comprehensive income (loss), tax | 11.2 | 6.0 | (9.0)
Other comprehensive income (loss) before reclassifications, after-tax | (35.5) | (1.1) | 52.7
Reclassification from accumulated other comprehensive income, after-tax | (5.8) | (22.2) | (20.1)
Adjustments for assumed changes in amortization patterns, after-tax | (0.7) | (0.1) |
Adjustments for assumed changes in policyholder liabilities, after-tax | 0.2 | 0.3 | 0.8
Other comprehensive income (loss) | (41.8) | (23.1) | 33.4
Net unrealized gains (losses) of derivative instruments including NCI |  |  |
Other Comprehensive Income (Loss) |  |  |
Reclassification from accumulated other comprehensive income, pre-tax | 0.6 |  |
Reclassification from accumulated other comprehensive income, after-tax | 0.6 |  |
Foreign currency translation adjustment relating to noncontrolling interest |  |  |
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss), pre-tax | 68.6 |  |
Other comprehensive income (loss), tax | 5.1 |  |
Other comprehensive income (loss) | 73.7 |  |
Liability for future policy benefits discount rate remeasurement gain (loss) |  |  |
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss) before reclassifications, pre-tax |  |  | 3,724.9
Other comprehensive income (loss), pre-tax | (365.9) | 6,030.4 |
Other comprehensive income (loss), tax | 53.2 | (1,251.9) | (885.2)
Other comprehensive income (loss) | (312.7) | 4,778.5 | 2,839.7
Market risk benefit nonperformance risk gain (loss) |  |  |
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss) before reclassifications, pre-tax |  |  | (0.6)
Other comprehensive income (loss), pre-tax | (39.2) | 144.3 |
Other comprehensive income (loss), tax | 8.3 | (30.2) | 0.2
Other comprehensive income (loss) | (30.9) | 114.1 | (0.4)
Foreign currency translation adjustment including NCI |  |  |
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss) before reclassifications, pre-tax |  |  | (259.6)
Other comprehensive income (loss), pre-tax | 68.0 | (26.9) | (240.3)
Other comprehensive income (loss) before reclassifications, tax |  |  | (5.6)
Other comprehensive income (loss), tax | 5.1 | 4.8 | (5.6)
Other comprehensive income (loss) before reclassifications, after-tax |  |  | (265.2)
Reclassification from accumulated other comprehensive income, after-tax |  |  | 19.3
Other comprehensive income (loss) | 73.1 | (22.1) | (245.9)
Foreign currency translation adjustment including NCI | Net realized capital gains (losses) |  |  |
Other Comprehensive Income (Loss) |  |  |
Reclassification from accumulated other comprehensive income, pre-tax |  |  | 19.3
Unrecognized postretirement benefit obligation including NCI |  |  |
Other Comprehensive Income (Loss) |  |  |
Other comprehensive income (loss) before reclassifications, pre-tax | (19.3) | 85.8 | 106.5
Other comprehensive income (loss), pre-tax | 3.1 | 122.5 | 158.3
Other comprehensive income (loss) before reclassifications, tax | 5.2 | (22.3) | (28.5)
Reclassification from accumulated other comprehensive income, tax | (5.9) | (9.6) | (14.0)
Other comprehensive income (loss), tax | (0.7) | (31.9) | (42.5)
Other comprehensive income (loss) before reclassifications, after-tax | (14.1) | 63.5 | 78.0
Reclassification from accumulated other comprehensive income, after-tax | 16.5 | 27.1 | 37.8
Other comprehensive income (loss) | 2.4 | 90.6 | 115.8
Unrecognized postretirement benefit obligation including NCI | Operating expense |  |  |
Other Comprehensive Income (Loss) |  |  |
Reclassification from accumulated other comprehensive income, pre-tax | $ 22.4 | $ 36.7 | $ 51.8

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