Schedule III - Real Estate and Accumulated Depreciation and Amortization - Under Direct Financing Leases (Details) - USD ($) $ in Thousands	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
Direct Financing Lease
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Land	1,064
Initial Cost to Company, Building, Improvements & Leasehold Interests	14,043
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 Warr Acres OK [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to Company, Building, Improvements & Leasehold Interests	1,365
Costs Capitalized Subsequent to Acquisition, Improvements	0
Costs Capitalized Subsequent to Acquisition, Carrying Costs	0
Direct Financing Lease | Dennys Stockton Ca [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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 [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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
Gross Amount at Which Carried at Close of Period, Land	0
Direct Financing Lease | Jared Jewelers Toledo Oh [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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 [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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 [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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 [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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 | Sunshine Energy Altamont Ks [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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
Direct Financing Lease | Walgreens Arlington VA [Member]
SEC Schedule, 12-28, Real Estate Companies, Investment in 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