Schedule III - Real Estate and Accumulated Depreciation (Reconciliation of Real Estate) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Real Estate, Beginning Balance	$ 3,031,139	$ 2,956,761	$ 2,538,270
Acquisitions	0	319,800	323,373
Capital expenditures	80,737	105,074	101,131
Other Deductions			(6,013)
Disposal of Assets	(207,804)	(350,496)
Real Estate, Ending Balance	2,904,072	3,031,139	2,956,761
SEC Schedule III, Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Real Estate Accumulated Depreciation, Beginning Balance	358,485	283,177	194,580
Accumulated Depreciation, Depreciation Expense	101,157	101,060	94,610
Other Deductions			(6,013)
Disposal of Assets	(12,020)	(25,752)
Real Estate Accumulated Depreciation, Ending Balance	447,622	$ 358,485	$ 283,177
Real Estate, Federal Income Tax Basis	$ 2,937,673