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Assets by Segment (Detail) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Segment Reporting Information |  |
Assets | $ 63,012.3 | $ 64,013.1
Operating Segments |  |
Segment Reporting Information |  |
Assets | 63,012.3 | 64,013.1
Operating Segments | Unum US |  |
Segment Reporting Information |  |
Assets | 18,036.3 | 18,109.1
Operating Segments | Unum UK |  |
Segment Reporting Information |  |
Assets | 3,497.6 | 3,428.1
Operating Segments | Colonial Life |  |
Segment Reporting Information |  |
Assets | 4,292.9 | 4,184.1
Operating Segments | Closed Block |  |
Segment Reporting Information |  |
Assets | 34,603.8 | 35,051.2
Operating Segments | Corporate and Other |  |
Segment Reporting Information |  |
Assets | $ 2,581.7 | $ 3,240.6

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