Schedule III - Real Estate and Accumulated Depreciation and Amortization - Real Estate - Held for Sale (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	$ 7,883,633	$ 7,314,132	$ 6,647,597	$ 5,913,547
Accumulated Depreciation and Amortization	1,016,271	$ 881,121	$ 742,467	$ 624,607
Assets Held-for-sale
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	10,837
Initial Cost to Company, Building, Improvements & Leasehold Interests	25,467
Costs Capitalized Subsequent to Acquisition, Improvements	680
Costs Capitalized Subsequent to Acquisition, Carrying Costs
Gross Amount at Which Carried at Close of Period, Land	8,606
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	11,897
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	20,503
Accumulated Depreciation and Amortization	6,897
Assets Held-for-sale | Amazing Jakes Plano Tx [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	5,705
Initial Cost to Company, Building, Improvements & Leasehold Interests	17,049
Costs Capitalized Subsequent to Acquisition, Improvements	18
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Gross Amount at Which Carried at Close of Period, Land	5,706
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	6,639
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	12,345
Accumulated Depreciation and Amortization	$ 5,098
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	35 years
Assets Held-for-sale | Chipotle Hadley Ma [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company, Land	45
Initial Cost to Company, Building, Improvements & Leasehold Interests	0
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Gross Amount at Which Carried at Close of Period, Land	632
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	0
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	$ 632
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	0 years
Assets Held-for-sale | Gander Mountain Eau Claire Wi [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company, Land	2,263
Initial Cost to Company, Building, Improvements & Leasehold Interests	8,418
Costs Capitalized Subsequent to Acquisition, Improvements	662
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Gross Amount at Which Carried at Close of Period, Land	2,263
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	5,258
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	7,521
Accumulated Depreciation and Amortization	$ 1,799
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	35 years
Assets Held-for-sale | Vacant Land Hadley Ma [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company, Land	2,824
Initial Cost to Company, Building, Improvements & Leasehold Interests	0
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Gross Amount at Which Carried at Close of Period, Land	5
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	0
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	$ 5
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	0 years