Rental and Other Revenues; Rental Property and Other Expenses (Details) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015	[1]	Jun. 30, 2015	[1]	Mar. 31, 2015	[1]	Dec. 31, 2014	[1]	Sep. 30, 2014	[1]	Jun. 30, 2014	[1]	Mar. 31, 2014	[1]	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Real Estate Revenue, Net [Abstract]
Contractual rents, net																	$ 513,909	$ 476,684	$ 437,855
Straight-line rental income, net																	22,054	21,045	16,653
Amortization of lease incentives																	(1,493)	(1,419)	(1,361)
Cost recovery income, net																	45,247	39,112	31,869
Lease termination fees																	990	658	1,436
Fee income																	3,717	3,313	5,282
Other miscellaneous operating revenues																	20,247	16,478	13,274
Real estate revenue, net	$ 160,126	[1]	$ 150,766		$ 148,543		$ 145,236		$ 140,724		$ 139,370		$ 139,956		$ 135,821		604,671	555,871	505,008
Future minimum base rents [Abstract]
2016	566,672																566,672
2017	539,352																539,352
2018	502,547																502,547
2019	437,797																437,797
2020	362,295																362,295
Thereafter	1,304,530																1,304,530
Total future minimum base rents	$ 3,713,193																3,713,193
Operating Costs and Expenses [Abstract]
Utilities, insurance and real estate taxes																	117,470	113,226	101,036
Maintenance, cleaning and general building																	79,091	74,109	67,319
Property management and administrative expenses																	12,446	12,093	11,515
Other miscellaneous operating expenses																	6,934	6,456	4,317
Rental property and other expenses, net																	$ 215,941	$ 205,884	$ 184,187

[1] As a result of discontinued operations, the amounts presented may not equal the amounts previously reported in the most recent Form 10-Qs or prior 10-K for each period. Below is a reconciliation to the amounts previously reported: Quarter Ended March 31, June 30, September 30, 2015 2015 2015Rental and other revenues, as reported$157,310 $161,136 $163,736Discontinued operations(12,074) (12,593) (12,970)Rental and other revenues, as adjusted$145,236 $148,543 $150,766Income from continuing operations, as reported$20,835 $26,891 $31,617Discontinued operations(3,915) (4,670) (4,265)Income from continuing operations, as adjusted$16,920 $22,221 $27,352Income from discontinued operations, as reported$— $— $—Additional discontinued operations from properties sold subsequent to the respective reporting period3,915 4,670 4,265Income from discontinued operations, as adjusted$3,915 $4,670 $4,265 Quarter Ended March 31, June 30, September 30, December 31, 2014 2014 2014 2014Rental and other revenues, as reported$148,453 $152,722 $152,629 $154,664Discontinued operations(12,632) (12,766) (13,259) (13,940)Rental and other revenues, as adjusted$135,821 $139,956 $139,370 $140,724Income from continuing operations, as reported$13,192 $24,512 $54,299 $23,585Discontinued operations(4,213) (4,138) (4,816) (5,434)Income from continuing operations, as adjusted$8,979 $20,374 $49,483 $18,151Income from discontinued operations, as reported$384 $— $— $—Additional discontinued operations from properties sold subsequent to the respective reporting period4,213 4,138 4,816 5,434Income from discontinued operations, as adjusted$4,597 $4,138 $4,816 $5,434