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
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%