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Reinsurance (Consolidated Reinsurance) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Premiums written [Abstract] |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | $ 33,137 | $ 31,543 | $ 19,879
Assumed |  |  |  |  |  |  |  |  | 3,239 | 3,440 | 3,932
Ceded |  |  |  |  |  |  |  |  | (7,132) | (6,838) | (6,098)
Net |  |  |  |  |  |  |  |  | 29,244 | 28,145 | 17,713
Premiums earned [Abstract] |  |  |  |  |  |  |  |  |  |  |
Direct |  |  |  |  |  |  |  |  | 32,782 | 31,811 | 19,355
Assumed |  |  |  |  |  |  |  |  | 3,332 | 3,744 | 3,676
Ceded |  |  |  |  |  |  |  |  | (7,080) | (6,806) | (5,818)
Net premiums earned | $ 7,218 | $ 7,807 | $ 7,237 | $ 6,772 | $ 7,059 | $ 7,688 | $ 7,405 | $ 6,597 | 29,034 | 28,749 | 17,213
Reinsurance recoveries on losses and loss expenses incurred |  |  |  |  |  |  |  |  | $ 5,500 | $ 4,100 | $ 3,100

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