Summary of Significant Accounting Policies - Properties in Development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Document Fiscal Year Focus	2017
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	$ 708,259
Other Cost and Expense, Operating	1,500	$ 1,500	$ 1,700
Refundable deposits - development
Property, Plant and Equipment [Line Items]
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	$ 3,500	$ 1,200