Schedule III - Real Estate and Accumulated Depreciation and Amortization - Under Direct Financing Leases (Details) - USD ($) $ in Thousands	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
Direct Financing Lease
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	1,064
Initial Cost to Company, Building, Improvements & Leasehold Interests	13,627
Costs Capitalized Subsequent to Acquisition, Improvements	1,984
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Gross Amount at Which Carried at Close of Period, Land	0
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	0
Accumulated Depreciation and Amortization	0
Direct Financing Lease | CVS Lafayette, LA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	949
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	0
Direct Financing Lease | Denny's Stockton, CA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	940
Initial Cost to Company, Building, Improvements & Leasehold Interests	509
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Food 4 Less Chula Vista, CA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	4,266
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Jared Jewelers Toledo, OH
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	1,458
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Jared Jewelers Lewisville, TX
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	1,503
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Jared Jewelers Glendale, AZ
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	1,599
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Rite Aid Kennett Square, PA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	0
Costs Capitalized Subsequent to Acquisition, Improvements	1,984
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Rite Aid Arlington, VA
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	3,201
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Sunshine Energy Altamont, KS
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	124
Initial Cost to Company, Building, Improvements & Leasehold Interests	142
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	$ 0