Schedule III - Real Estate and Accumulated Depreciation and Amortization - Real Estate - Held for Sale (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	$ 7,314,132	$ 6,647,597	$ 5,913,547	$ 5,236,251
Accumulated Depreciation and Amortization	881,121	$ 742,467	$ 624,607	$ 513,175
Assets Held-for-sale
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	4,834
Initial Cost to Company, Building, Improvements & Leasehold Interests	2,882
Costs Capitalized Subsequent to Acquisition, Improvements	370
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Gross Amount at Which Carried at Close of Period, Land	2,226
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	2,743
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	4,969
Accumulated Depreciation and Amortization	886
Assets Held-for-sale | Bombones Sports Bar - Dallas, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	1,138
Initial Cost to Company, Building, Improvements & Leasehold Interests	1,025
Costs Capitalized Subsequent to Acquisition, Improvements	370
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Gross Amount at Which Carried at Close of Period, Land	1,138
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	1,309
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	2,447
Accumulated Depreciation and Amortization	$ 447
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	40 years
Assets Held-for-sale | Chipotle Hadley Ma
SEC Schedule III, 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	611
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	$ 611
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	0 years
Assets Held-for-sale | SunTrust St. Petersburg, FL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company, Land	207
Initial Cost to Company, Building, Improvements & Leasehold Interests	1,150
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	207
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	1,150
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	1,357
Accumulated Depreciation and Amortization	$ 155
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	30 years
Assets Held-for-sale | Vacant Land Hadley Ma
SEC Schedule III, 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
Assets Held-for-sale | Vacant Property Burton, MI
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost to Company, Land	620
Initial Cost to Company, Building, Improvements & Leasehold Interests	707
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	265
Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	284
Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	549
Accumulated Depreciation and Amortization	$ 284
Life on Which Depreciation & Amortization in Latest Income Statement is Computed	40 years