Segment Reporting - Operating Activity Within Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Total office revenues	$ 167,608	$ 136,225	$ 475,083	$ 404,472
Office expenses	(56,926)	(49,195)	(158,190)	(139,936)
Total multifamily revenues	24,513	23,852	72,825	70,871
Multifamily expenses	(5,950)	(6,191)	(17,322)	(17,941)
Segment profit	129,245	104,691	372,396	317,466
Operating Segments [Member] | Office Segment [Member]
Segment Reporting Information [Line Items]
Total office revenues	167,608	136,225	475,083	404,472
Office expenses	(56,926)	(49,195)	(158,190)	(139,936)
Segment profit	110,682	87,030	316,893	264,536
Operating Segments [Member] | Multifamily Segment [Member]
Segment Reporting Information [Line Items]
Total multifamily revenues	24,513	23,852	72,825	70,871
Multifamily expenses	(5,950)	(6,191)	(17,322)	(17,941)
Segment profit	$ 18,563	$ 17,661	$ 55,503	$ 52,930