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Stock Option and Incentive Plan Stock Option and Incentive Plan (Narrative) (Details) $ in Millions | Feb. 06, 2018 USD ($) yr indices | Feb. 04, 2018 USD ($) shares
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2018 MYLTIP [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
TSR Measurement, Years | yr | 3 |
Indices Used to Compare TSR | indices | 2 |
Target Tier | $ 16.2 |
Percentage of annualized TSR for Reduction of Earned Awards | 0.00% |
Percentage to Cause Some Awards to be Earned Even if on a Relative Basis it Would Not Result in any Earned Awards | 12.00% |
Distributions Percent Before Measurement Date | 10.00% |
Value of MYLTIP Awards | $ 13.3 |
MYLTIP Value Amortized Into Earnings, Years | yr | 4 |
2015 MYLTIP [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Value of MYLTIP Awards |  | $ 3.6
Final awards percentage of target |  | 22.00%
2015 MYLTIP Units Forfeited | shares |  | 337,847
Cohen & Steers Realty Majors Portfolio Index [Member] | 2018 MYLTIP [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Weighted Percentage of Index Used to Compare to TSR | 50.00% |
Nareit Office Index adjusted [Member] | 2018 MYLTIP [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Weighted Percentage of Index Used to Compare to TSR | 50.00% |
Maximum [Member] | 2018 MYLTIP [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Potential Awards Earned | $ 32.3 |
MYLTIP vesting 2022 [Member] | 2018 MYLTIP [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Vesting Percentage | 50.00% |
MYLTIP vesting 2021 [Member] | 2018 MYLTIP [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Vesting Percentage | 50.00% |

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