Investments in Real Estate (Details) ft² in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) ft² item	Dec. 31, 2014 USD ($) ft² item	Dec. 31, 2013 USD ($)
Real estate properties
Leasable square feet | ft²	76.0
Assumed mortgages payable	$ 646,740,000	$ 852,575,000
Total revenue	1,023,285,000	933,505,000	$ 780,209,000
Income from continuing operations	284,855,000	269,140,000	180,613,000
Total capitalized costs on existing properties	22,978,169
Intangible value of the in-place leases acquired	1,056,715,000	1,005,244,000
Intangible value of above-market leases acquired	304,548,000	252,581,000
In-place lease, depreciation and amortization to expense	87,900,000	83,600,000	65,500,000
Net Decrease to Rental Income for Capitalized above Market and below Market Leases	7,900,000	8,000,000	$ 8,200,000
Net increase (decrease) to rental revenue
2016	(8,075,000)
2017	(8,019,000)
2018	(7,771,000)
2019	(6,781,000)
2020	(6,108,000)
Thereafter	45,569,000
Totals	8,815,000
Increase to amortization expense
2016	89,858,000
2017	88,669,000
2018	86,174,000
2019	76,109,000
2020	70,915,000
Thereafter	380,591,000
Totals	$ 792,316,000
Maximum
Real estate properties
Investments to any one tenant as a percentage of total assets	10.00%
New properties and properties under development
Real estate properties
Real estate acquired	$ 1,260,000,000	$ 1,400,000,000
Number of properties | item	286	506
Initial weighted average contractual lease rate of properties acquired (as a percent)	6.60%	7.10%
Number of states in which properties are located | item	40	42
Leasable square feet | ft²	6.2	9.8
Leased area (as a percent)	100.00%	100.00%
Weighted average lease term	16 years 6 months	12 years 9 months 18 days
Number of industries in which tenants operate | item	21	32
Acquisition transaction costs	$ 913,000	$ 453,000
New properties and properties under development | Retail properties
Real estate properties
Property type- percentage of properties acquired	87.30%	85.70%
New properties and properties under development | Office properties
Real estate properties
Property type- percentage of properties acquired		6.40%
New properties and properties under development | Industrial and distribution properties
Real estate properties
Property type- percentage of properties acquired	12.70%	6.60%
New properties and properties under development | Manufacturing properties
Real estate properties
Property type- percentage of properties acquired		1.30%
Properties under development or expansion
Real estate properties
Number of properties | item	35	40
Initial weighted average contractual lease rate of properties acquired (as a percent)	9.70%	8.40%
Investments in properties	$ 45,800,000	$ 81,900,000
Aggregate acquisitions
Real estate properties
Real estate acquired		1,400,000,000
Land recorded related to acquisition	257,100,000	295,600,000
Buildings and improvements recorded	937,100,000	984,100,000
Intangible assets related to leases recorded	105,800,000	209,400,000
Other assets recorded		901,000
Intangible liabilities related to leases and other assumed liabilities recorded	40,900,000	87,400,000
Premiums recorded upon acquisition of mortgages		604,000
Contingent consideration associated with acquisition	0	0
Total revenue	43,400,000	75,100,000
Income from continuing operations	21,100,000	27,800,000
Amount of investment allocated by estimate	195,400,000
Amount of investment previously allocated by estimate finalized during the period	147,100,000
Investments in Existing Properties
Real estate properties
Re-leasing costs and real estate improvements included in capitalized costs on existing properties	748,000
Recurring Capital Expenditures Real Estate Improvements	7,600,000
Nonrecurring Building Real Estate Improvements	3,200,000
Total capitalized costs on existing properties	11,500,000	6,000,000
Real estate investments with existing leases
Real estate properties
Real estate acquired	$ 391,400,000	$ 957,400,000
Number of properties | item	86	201