Schedule III - Real Estate and Accumulated Depreciation and Amortization Real Estate - Held for Sale (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	$ 5,913,547	$ 5,236,251	$ 4,686,844	$ 4,145,368
Real Estate and Accumulated Depreciation, Accumulated Depreciation and Amortization	624,607	$ 513,175	$ 418,136	$ 333,778
Assets Held-for-sale
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	12,041
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	27,882
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	9,419
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	27,666
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	37,085
Real Estate and Accumulated Depreciation, Accumulated Depreciation and Amortization	4,419
Assets Held-for-sale | BJ's Wholesale Club Attleboro, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	4,988
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	26,364
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	4,987
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	26,364
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	31,351
Real Estate and Accumulated Depreciation, Accumulated Depreciation and Amortization	$ 3,771
Real Estate and Accumulated Depreciation, Life on Which Depreciation & Amortization in Latest Income Statement is Computed	30 years
Assets Held-for-sale | CarQuest Portland, ME
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	$ 0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	123
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	264
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	123
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	264
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	387
Real Estate and Accumulated Depreciation, Accumulated Depreciation and Amortization	$ 86
Real Estate and Accumulated Depreciation, Life on Which Depreciation & Amortization in Latest Income Statement is Computed	15 years
Assets Held-for-sale | Chipotle Hadley, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	$ 0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	45
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	497
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	497
Assets Held-for-sale | Fuel-On Hazleton, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	2,529
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	728
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	2,529
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	512
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	3,041
Real Estate and Accumulated Depreciation, Accumulated Depreciation and Amortization	$ 377
Real Estate and Accumulated Depreciation, Life on Which Depreciation & Amortization in Latest Income Statement is Computed	20 years
Assets Held-for-sale | Taco Bell Ormond Beach, FL
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	$ 0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	632
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	526
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	632
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	526
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	1,158
Real Estate and Accumulated Depreciation, Accumulated Depreciation and Amortization	$ 185
Real Estate and Accumulated Depreciation, Life on Which Depreciation & Amortization in Latest Income Statement is Computed	40 years
Assets Held-for-sale | Vacant Land Rockwall, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	$ 0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	900
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	545
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	545
Assets Held-for-sale | Vacant Land Hadley, MA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Real Estate and Accumulated Depreciation, Encumbrances	0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Land	2,824
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land	106
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Building, Improvements & Leasehold Interests	0
Real Estate and Accumulated Depreciation, Gross Amount at Which Carried at Close of Period, Land, Building, Improvements & Leasehold Interests	$ 106