Rental and Other Revenues; Rental Property and Other Expenses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate Revenue, Net [Abstract]
Contractual rents, net									$ 583,825	$ 561,242	$ 513,909
Straight-line rental income, net									31,721	23,909	22,054
Cost recovery income, net									53,942	48,730	45,247
Lease termination fees									1,501	2,311	990
Other miscellaneous operating revenues									31,748	29,442	22,471
Real estate revenue, net	$ 175,861	$ 180,185	$ 177,283	$ 169,408	$ 167,646	$ 166,269	$ 166,860	$ 164,859	702,737	665,634	604,671
Future minimum base rents [Abstract]
2018	595,638								595,638
2019	556,405								556,405
2020	490,429								490,429
2021	432,602								432,602
2022	377,964								377,964
Thereafter	1,378,404								1,378,404
Total future minimum base rents	$ 3,831,442								3,831,442
Operating Costs and Expenses [Abstract]
Utilities, insurance and real estate taxes									126,461	124,940	117,470
Maintenance, cleaning and general building									90,764	86,221	79,091
Property management and administrative expenses									13,117	12,588	12,446
Other miscellaneous operating expenses									6,546	7,336	6,934
Rental property and other expenses, net									$ 236,888	$ 231,085	$ 215,941