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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  | 3 Months Ended |  |  | 3 Months Ended
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2009 Accounting Standards Update 2010-26 | Mar. 31, 2013 Aflac Japan | Mar. 31, 2012 Aflac Japan | Dec. 31, 2012 Aflac Japan | Dec. 31, 2009 Retained earnings: Accounting Standards Update 2010-26 | Dec. 31, 2009 Accumulated Other Comprehensive Income (Loss) Accounting Standards Update 2010-26 | Mar. 31, 2013 Dollar denominated investment portfolio Aflac Japan | Mar. 31, 2012 Dollar denominated investment portfolio Aflac Japan
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Income tax expense (benefit) related to items of other comprehensive income (loss) | $ (58) | $ 311 |  |  |  |  |  |  | $ 424 | $ 157
Aflac Japan's percentage of the Company's total revenues |  |  |  | 75.00% | 77.00% |  |  |  |  |
Aflac Japan's percentage of the Company's total assets |  |  |  | 86.00% |  | 87.00% |  |  |  |
Effective income tax rate excluding income tax expense (benefit) related to Aflac Japan's dollar-denominated portfolio |  |  |  |  |  |  |  |  | 43.00% | 44.50%
Adjustment to shareholders' equity upon adoption of new DAC accounting guidance |  |  | $ (458) |  |  |  | $ (391) | $ (67) |  |

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