Share-Based Compensation (Multi-Year Outperformance Plan Agreement) (Details) - USD ($)				3 Months Ended		6 Months Ended
		Apr. 15, 2015	Apr. 15, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payments of Dividends						$ 37,305,000	$ 28,830,000
New Multi-Year Outperformance Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation				$ 2,000,000	$ 1,800,000.0	$ 2,200,000	$ 1,801,603
New Multi-Year Outperformance Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Opp units issued (in shares)			$ 8,880,579
Company's market capitalization percentage			5.00%
Share-based payment award, award vesting rights						33.33%
Distribution entitlement percentage						10.00%
Payments of Dividends				$ 300,000		$ 400,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period		367,059
New Multi-Year Outperformance Plan | Performance Shares | Performance Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Absolute Component: 4% of any excess Total Return attained above an absolute hurdle measured from the beginning of such period:				21.00%		21.00%
100% will be earned if cumulative Total Return achieved is at least:	[1]			18.00%		18.00%
50% will be earned if cumulative Total Return achieved is:	[1]			0.00%		0.00%
0% will be earned if cumulative Total Return achieved is less than:	[1]			0.00%		0.00%
New Multi-Year Outperformance Plan | Performance Shares | Annual Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Absolute Component: 4% of any excess Total Return attained above an absolute hurdle measured from the beginning of such period:				7.00%		7.00%
100% will be earned if cumulative Total Return achieved is at least:	[1]			6.00%		6.00%
50% will be earned if cumulative Total Return achieved is:	[1]			0.00%		0.00%
0% will be earned if cumulative Total Return achieved is less than:	[1]			0.00%		0.00%
New Multi-Year Outperformance Plan | Performance Shares | Interim Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Absolute Component: 4% of any excess Total Return attained above an absolute hurdle measured from the beginning of such period:				14.00%		14.00%
100% will be earned if cumulative Total Return achieved is at least:	[1]			12.00%		12.00%
50% will be earned if cumulative Total Return achieved is:	[1]			0.00%		0.00%
0% will be earned if cumulative Total Return achieved is less than:	[1]			0.00%		0.00%
Relative Component | Excess Return, Above Peer Group | New Multi-Year Outperformance Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized, percentage of benchmark				4.00%		4.00%
Relative Component | Cumulative Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded as a percentage of maximum				100.00%		100.00%
Relative Component | Cumulative Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares | Minimum | Performance Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Return percentage threshold				0.00%		0.00%
Relative Component | Cumulative Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares | Minimum | Annual Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Return percentage threshold				0.00%		0.00%
Relative Component | Cumulative Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares | Minimum | Interim Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Return percentage threshold				0.00%		0.00%
Relative Component | Cumulative Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares | Maximum | Performance Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Return percentage threshold				18.00%		18.00%
Relative Component | Cumulative Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares | Maximum | Annual Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Return percentage threshold				6.00%		6.00%
Relative Component | Cumulative Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares | Maximum | Interim Period
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Return percentage threshold				12.00%		12.00%
Relative Component | Cumulative Return, Equal to Threshold | New Multi-Year Outperformance Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded as a percentage of maximum				50.00%		50.00%
Relative Component | Cumulative Return, Below Threshold | New Multi-Year Outperformance Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded as a percentage of maximum				0.00%		0.00%
Absolute Component | Excess Return, Above Threshold | New Multi-Year Outperformance Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized, percentage of benchmark				4.00%		4.00%

[1] The “Peer Group” is comprised of the companies in the SNL US REIT Office Index.