Property, net: (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Land	$ 2,238,048	$ 2,242,291
Buildings and improvements	9,524,581	9,479,337
Tenant improvements	628,014	600,436
Equipment and furnishings	166,632	152,554
Construction in progress	409,631	303,264
Total	12,966,906	12,777,882
Less accumulated depreciation	(1,935,430)	(1,709,992)
Property, net	$ 11,031,476	$ 11,067,890