Rental and Other Revenues; Rental Property and Other Expenses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Real Estate Revenue, Net [Abstract]
Contractual rents, net									$ 474,163	$ 413,650	$ 374,674
Straight-line rental income, net									17,226	16,104	11,536
Amortization of lease incentives									(1,409)	(1,389)	(1,302)
Cost recovery income, net									43,586	38,547	32,602
Lease termination fees									2,030	1,848	1,802
Fee income									5,557	4,965	5,571
Other miscellaneous operating revenues									15,657	11,321	7,073
Real estate revenue, net	148,988	144,827	132,618	130,377	126,316	120,456	120,423	117,851	556,810	485,046	431,956
Future minimum base rents [Abstract]
2014	512,083								512,083
2015	486,862								486,862
2016	438,741								438,741
2017	375,264								375,264
2018	311,015								311,015
Thereafter	1,111,421								1,111,421
Total future minimum base rents	3,235,386								3,235,386
Operating Costs and Expenses [Abstract]
Utilities, insurance and real estate taxes									108,095	94,710	86,007
Maintenance, cleaning and general building									75,969	65,430	55,260
Property management and administrative expenses									12,429	11,718	10,492
Other miscellaneous operating expenses									6,851	4,886	5,547
Rental property and other expenses, net									$ 203,344	$ 176,744	$ 157,306