Dispositions, Assets Held for Sale and Discontinued Operations (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Rental income, disposed	$ 120,681	$ 401,742	$ 435,404
Expenses:
Interest expense, disposed	6,570	47,083	68,978
Property operating expenses, disposed	12,402	20,847	22,313
Depreciation expense, disposed	31,736	98,949	114,869
Expenses, disposed	50,708	166,879	206,160
Income (loss) from disposed properties	$ 69,973	$ 234,863	$ 229,244