Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Schedule of Equity Method Investments [Line Items]
Total revenue		$ 661,151	$ 632,228
Expenses
Depreciation and amortization		165,797	159,205
Total expenses		452,999	426,003
Operating income		208,152	206,225
Other expense
Interest expense		90,220	95,534
Net income		216,312	115,564
Income from unconsolidated joint ventures		461	3,084
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue	[1]	56,486	54,761
Expenses
Operating		22,849	22,079
Depreciation and amortization		14,725	14,309
Total expenses		37,574	36,388
Operating income		18,912	18,373
Other expense
Interest expense		14,424	9,300
Net income		4,488	9,073
Company's share of net income		1,826	4,323
Basis differential	[2]	(1,365)	(1,239)
Income from unconsolidated joint ventures		461	3,084
Straight-line rent adjustments		1,800	7,000
Colorado Center [Member] | Unconsolidated Joint Ventures [Member]
Other expense
Straight-line rent adjustments		700	700
"Above" and "below" market rent adjustments, net		$ 400	$ 400

[1] Includes straight-line rent adjustments of approximately $1.8 million and $7.0 million for the three months ended March 31, 2018 and 2017, respectively.

[2] Includes straight-line rent adjustments of approximately $0.7 million and $0.7 million for the three months ended March 31, 2018 and 2017, respectively. Also includes net above-/below-market rent adjustments of approximately $0.4 million and $0.4 million for the three months ended March 31, 2018 and 2017, respectively.