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Capital Ratios (Details) - USD ($) $ in Thousands | Jan. 01, 2016 | Jun. 30, 2018 | May 24, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jul. 31, 2013
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Total Capital (to Risk Weighted Assets), Actual Ratio (as a percent) |  | 20.04% |  | 19.51% |  |
Total Capital (to Risk Weighted Assets), For Capital Adequacy Purposes Ratio (as a percent) | 0.625% |  |  |  | 2.50% |
Amount of assets to exempt banks |  | $ 11,988,995 |  | $ 12,184,698 |  |
CET 1 ratio |  | 17.75% |  | 17.11% |  |
Tier 1 capital (to Average Assets), Actual Ratio (as a percent) |  | 15.32% |  | 14.62% |  |
Tier 1 capital (to Risk Weighted Assets), Actual Ratio (as a percent) |  | 19.32% |  | 18.73% |  | 6.00%
Term of mandatory compliance | 4 years |  |  |  |  |
Minimum |  |  |  |  |  |
Community bank leverage ratio |  |  | 8.00% |  |  |
CET 1 ratio |  | 7.00% |  |  |  |
Tier 1 capital (to Risk Weighted Assets), Actual Ratio (as a percent) |  |  |  |  | 4.50% | 4.00%
Maximum |  |  |  |  |  |
Amount of assets to exempt banks |  |  | $ 10,000,000 |  |  |
Community bank leverage ratio |  |  | 10.00% |  |  |

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