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Share-Based Payments Share-based Payments - Vesting Criteria for Performance and Phantom Shares (Details) | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
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Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |  |  |  |  |
Increase in BVPS | 0.00% | 5.90% | 10.70% | 13.30% | 6.20%

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- Definition Share-based Compensation Arrangement by Share-based Award, Entities That Calculate Net Asset Value Per Share, Percent + References No definition available. + Details
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