Discontinued Operations - Summary of Revenue and Expense Information for Properties Sold which Qualify for Discontinued Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue:
Rents									$ 486,731	$ 483,682	$ 461,387
Tenant reimbursements									85,722	84,879	79,087
Other									6,617	3,221	4,186
Total revenue	$ 153,992	$ 152,585	$ 145,648	$ 150,406	$ 147,810	$ 146,558	$ 150,500	$ 152,114	602,631	596,982	562,210
Expenses:
Property operating expenses									181,170	177,330	160,406
Real estate taxes									50,623	51,844	55,612
Depreciation and amortization									219,029	208,569	197,021
Total operating expenses									568,730	473,078	446,418
Interest income									1,224	3,974	1,044
Income from discontinued operations before gain on sale of interests in real estate									0	18	825
Net gain on disposition of discontinued operations									0	900	3,382
Total discontinued operations									$ 0	918	4,207
Discontinued Operations [Member]
Revenue:
Rents										0	4,754
Tenant reimbursements										26	355
Other										0	123
Total revenue										26	5,232
Expenses:
Property operating expenses										8	1,839
Real estate taxes										0	649
Depreciation and amortization										0	1,921
Total operating expenses										8	4,409
Interest income										0	2
Income from discontinued operations before gain on sale of interests in real estate										18	825
Net gain on disposition of discontinued operations										900	3,382
Total discontinued operations										$ 918	$ 4,207