Schedule III - Real Estate and Accumulated Depreciation and Amortization Under Direct Financing Leases (Details) - USD ($) $ in Thousands	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
Direct Financing Lease
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	2,299
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	19,666
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	1,984
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	0
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	0
Real Estate and Accumulated Depreciation, Accumulated Depreciation and Amortization	0
Direct Financing Lease | CVS Lafayette, LA
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	0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	949
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	0
Direct Financing Lease | CVS Oklahoma City, OK 1
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, Building, Improvements & Leasehold Interests	1,365
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
Direct Financing Lease | CVS Oklahoma City, OK 2
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, Building, Improvements & Leasehold Interests	1,419
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
Direct Financing Lease | Denny's Stockton, CA
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	940
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	509
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
Direct Financing Lease | Food 4 Less Chula Vista, CA
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	0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	4,266
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	0
Direct Financing Lease | Jared Jewelers Toledo, OH
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, Building, Improvements & Leasehold Interests	1,458
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
Direct Financing Lease | Jared Jewelers Lewisville, 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, Building, Improvements & Leasehold Interests	1,503
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
Direct Financing Lease | Jared Jewelers Glendale, AZ
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, Building, Improvements & Leasehold Interests	1,599
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
Direct Financing Lease | Kash n' Karry Valrico, 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	1,235
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	3,255
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
Direct Financing Lease | Rite Aid Kennett Square, 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, Building, Improvements & Leasehold Interests	0
Real Estate and Accumulated Depreciation, Costs Capitalized Subsequent to Acquisition, Improvements	1,984
Real Estate and Accumulated Depreciation, 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]
Real Estate and Accumulated Depreciation, Encumbrances	0
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	3,201
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
Direct Financing Lease | Sunshine Energy Altamont, KS
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	124
Real Estate and Accumulated Depreciation, Initial Cost to Company, Building, Improvements & Leasehold Interests	142
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