Rental and Other Revenues; Rental Property and Other Expenses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate Revenue, Net [Abstract]
Contractual rents, net									$ 512,766	$ 474,163	$ 413,650
Straight-line rental income, net									21,281	17,226	16,104
Amortization of lease incentives									(1,465)	(1,409)	(1,389)
Cost recovery income, net									51,273	43,586	38,547
Lease termination fees									2,203	2,030	1,848
Fee income									3,560	5,557	4,965
Other miscellaneous operating revenues									18,850	15,657	11,321
Real estate revenue, net	154,664	152,629	152,722	148,453	148,988	144,827	132,618	130,377	608,468	556,810	485,046
Future minimum base rents [Abstract]
2015	540,981								540,981
2016	521,790								521,790
2017	472,532								472,532
2018	422,418								422,418
2019	364,170								364,170
Thereafter	1,304,381								1,304,381
Total future minimum base rents	3,626,272								3,626,272
Operating Costs and Expenses [Abstract]
Utilities, insurance and real estate taxes									120,380	108,095	94,710
Maintenance, cleaning and general building									82,983	75,969	65,430
Property management and administrative expenses									13,110	12,429	11,718
Other miscellaneous operating expenses									9,031	6,851	4,886
Rental property and other expenses, net									$ 225,504	$ 203,344	$ 176,744