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Employee Share-Based Incentive Compensation Plans (Tables) | 12 Months Ended
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Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Summary of performance share activity | The following table presents performance share activity for the years ended December 31, 2017, 2016 and 2015 for performance shares granted under the WTM Incentive Plan:
|  | Year Ended December 31,
|  | 2017 |  | 2016 |  | 2015
$ in millions |  | Target Performance Shares Outstanding |  | Accrued Expense |  | Target Performance Shares Outstanding |  | Accrued Expense |  | Target Performance Shares Outstanding |  | Accrued Expense
Beginning of period |  | 80,353 |  |  | $ | 42.4 |  |  | 93,654 |  |  | $ | 57.7 |  |  | 111,257 |  |  | $ | 44.4 |
Shares paid or expired (1) |  | (30,838 | ) |  | (21.9 | ) |  | (36,294 | ) |  | (41.0 | ) |  | (42,959 | ) |  | (30.8 | )
New grants |  | 17,710 |  |  | — |  |  | 22,615 |  |  | — |  |  | 29,195 |  |  | — |
Forfeitures (2) |  | (16,710 | ) |  | (9.3 | ) |  | 378 |  |  | .5 |  |  | (3,839 | ) |  | (.3 | )
Expense recognized |  | — |  |  | 34.6 |  |  | — |  |  | 25.2 |  |  | — |  |  | 44.4 |
End of period (3) |  | 50,515 |  |  | $ | 45.8 |  |  | 80,353 |  |  | $ | 42.4 |  |  | 93,654 |  |  | $ | 57.7 |
(1) | WTM performance share payments in 2017 for the 2014-2016 performance cycle, which were paid in March 2017 ranged from 34% to 76% of target. WTM performance shares payments in 2016 for the 2013-2015 performance cycle ranged from 140% to 142% of target. WTM performance shares payments in 2015 for the 2012-2014 performance cycle ranged from 91% to 145.5% of target.
(2) | Amounts include changes in assumed forfeitures, as required under GAAP.
(3) | Outstanding performance share awards as of December 31, 2017, 2016 and 2015 exclude 2,195, 7,315 and 10,826 unvested performance shares awards for employees of Sirius Group.
Summary of performance shares outstanding and accrued expense for performance shares awarded under an Incentive Plan | following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2017 for each performance cycle:
$ in millions |  | Target WTM Performance Shares Outstanding |  | Accrued Expense
Performance cycle: |  |  |  |  |  |
2017 – 2019 |  | 16,680 |  |  | $ | 8.3 |
2016 – 2018 |  | 16,235 |  |  | 14.7 |
2015 – 2017 |  | 18,370 |  |  | 23.6 |
Sub-total |  | 51,285 |  |  | 46.6 |
Assumed forfeitures |  | (770 | ) |  | (.8 | )
Total |  | 50,515 |  |  | $ | 45.8 |
Schedule of Nonvested Restricted Stock Activity | The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2017, 2016 and 2015:
|  | Year Ended December 31,
|  | 2017 |  | 2016 |  | 2015
$ in millions |  | Restricted Shares |  | Unamortized Issue Date Fair Value |  | Restricted Shares |  | Unamortized Issue Date Fair Value |  | Restricted Shares |  | Unamortized Issue Date Fair Value
Non-vested, |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period |  | 70,620 |  |  | $ | 19.7 |  |  | 70,675 |  |  | $ | 15.7 |  |  | 83,314 |  |  | $ | 14.3 |
Issued |  | 17,985 |  |  | 16.3 |  |  | 25,365 |  |  | 20.2 |  |  | 23,640 |  |  | 15.7 |
Vested |  | (28,846 | ) |  | — |  |  | (24,620 | ) |  | — |  |  | (36,279 | ) |  | — |
Forfeited |  | (6,004 | ) |  | (3.5 | ) |  | (800 | ) |  | (.3 | ) |  | — |  |  | — |
Expense recognized |  | — |  |  | (18.2 | ) |  | — |  |  | (15.9 | ) |  | — |  |  | (14.3 | )
End of period (1) |  | 53,755 |  |  | $ | 14.3 |  |  | 70,620 |  |  | $ | 19.7 |  |  | 70,675 |  |  | $ | 15.7 |
(1) | Outstanding restricted share awards as of December 31, 2017, 2016 and 2015 include 2,195, 5,235, and 9,205 unvested restricted shares for employees of Sirius Group.

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