Schedule III - Real Estate and Accumulated Depreciation - Rollforward of Accumulated Depreciation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Balance as of the beginning of the year	$ 581,505	$ 474,223	$ 383,966
Additions	171,545	117,282	101,194
Dispositions	(18,267)	(10,000)	(10,937)
Balance as of the end of the year	734,783	581,505	474,223
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Accumulated depreciation lease intangibles	$ 286,900	$ 236,100	$ 201,900
Tenant Improvements | Minimum
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Estimated useful life	1 month
Tenant Improvements | Maximum
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Estimated useful life	193 months
Furniture and Fixtures
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Estimated useful life	5 years