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Investments - Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | $ 78 | $ 135 | $ 106
Realized - impairments | (73) | (116) | (125)
Total realized gains (losses) on securities | 5 | 19 | (19)
Net of tax and noncontrolling interests |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | 51 | 85 | 66
Realized - impairments | (48) | (72) | (78)
Total realized gains (losses) on securities | 3 | 13 | (12)
Change in unrealized | 436 | 72 | (411)
Total pretax |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | 78 | 135 | 106
Realized - impairments | (73) | (116) | (125)
Total realized gains (losses) on securities | 5 | 19 | (19)
Change in unrealized | 441 | 105 | (645)
Fixed maturities |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | 17 | 36 | 19
Realized - impairments | (20) | (38) | (43)
Total realized gains (losses) on securities | (3) | (2) | (24)
Change in unrealized | 532 | 90 | (941)
Equity securities |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | 70 | 106 | 94
Realized - impairments | (64) | (93) | (94)
Total realized gains (losses) on securities | 6 | 13 | 0
Change in unrealized | 128 | 67 | (134)
Mortgage loans and other investments |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | (6) | 0 | (2)
Realized - impairments | (4) | 0 | (3)
Total realized gains (losses) on securities | (10) | 0 | (5)
Change in unrealized | 0 | 0 | 0
Other, Including DPAC and reserves on annuities and long-term care |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | (3) | (7) | (5)
Realized - impairments | 15 | 15 | 15
Total realized gains (losses) on securities | 12 | 8 | 10
Change in unrealized | (219) | (52) | 430
Tax effects |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | (27) | (48) | (38)
Realized - impairments | 25 | 41 | 45
Total realized gains (losses) on securities | (2) | (7) | 7
Change in unrealized | (5) | (37) | 226
Noncontrolling interests |  |  |
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments |  |  |
Realized before impairments | 0 | (2) | (2)
Realized - impairments | 0 | 3 | 2
Total realized gains (losses) on securities | 0 | 1 | 0
Change in unrealized | $ 0 | $ 4 | $ 8

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