Investments in Real Estate (Details) ft² in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) ft²	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2016 USD ($) ft² property state item	Sep. 30, 2015 USD ($) ft² item
Real estate properties
Leasable square feet | ft²	80.5		80.5
Number of investments in properties that caused any tenant to be 10% of more of total assets | item			0
Total revenue	$ 277,174,000	$ 258,889,000	$ 815,329,000	$ 759,617,000
In-place lease, depreciation and amortization to expense			69,600,000	65,500,000
Net Decrease to Rental Income for Capitalized above Market and below Market Leases			6,700,000	5,800,000
Net increase (decrease) to rental revenue
2016	(2,512,000)		(2,512,000)
2017	(10,006,000)		(10,006,000)
2018	(9,758,000)		(9,758,000)
2019	(8,803,000)		(8,803,000)
2020	(8,042,000)		(8,042,000)
Thereafter	24,553,000		24,553,000
Totals	(14,568,000)		(14,568,000)
Increase to amortization expense
2016	24,248,000		24,248,000
2017	95,904,000		95,904,000
2018	93,409,000		93,409,000
2019	83,084,000		83,084,000
2020	77,569,000		77,569,000
Thereafter	433,821,000		433,821,000
Totals	$ 808,035,000		$ 808,035,000
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			$ 1,100,000,000	$ 1,100,000,000
Number of properties			236	195
Initial weighted average contractual lease rate of properties acquired (as a percent)			6.40%	6.50%
Number of states in which properties are located			36	36
Leasable square feet | ft²	5.2	5.1	5.2	5.1
Leased area (as a percent)	100.00%	100.00%	100.00%	100.00%
Weighted average lease term			15 years	16 years 8 months 12 days
Number of industries in which tenants operate | item			24	18
Acquisition transaction costs			$ 119,000	$ 368,000
New properties and properties under development | Retail properties
Real estate properties
Property type- percentage of properties acquired			80.70%	87.00%
New properties and properties under development | Industrial and distribution properties
Real estate properties
Property type- percentage of properties acquired			19.30%	13.00%
Properties under development or expansion
Real estate properties
Number of properties | item			30	30
Initial weighted average contractual lease rate of properties acquired (as a percent)			7.10%	9.90%
Investments in properties			$ 87,700,000	$ 37,100,000
Aggregate acquisitions
Real estate properties
Real estate acquired			1,100,000,000	1,100,000,000
Land recorded related to acquisition			267,800,000	214,800,000
Buildings and improvements recorded			691,900,000	780,800,000
Intangible assets related to leases recorded			140,400,000	86,600,000
Intangible liabilities related to leases and other assumed liabilities recorded			26,300,000	27,200,000
Premiums recorded upon acquisition of mortgages			692,000
Total revenue			22,500,000	25,300,000
Net income			11,200,000	12,600,000
Amount of investment allocated by estimate			401,100,000
Amount of investment previously allocated by estimate finalized during the period			195,400,000
Investments in Existing Properties
Real estate properties
Total capitalized costs on existing properties	$ 5,300,000	$ 5,900,000	5,300,000	5,900,000
Re-leasing costs and real estate improvements included in capitalized costs on existing properties			564,000
Recurring Capital Expenditures Real Estate Improvements			486,000
Nonrecurring Building Real Estate Improvements			4,200,000
Real estate investments with existing leases
Real estate properties
Real estate acquired			$ 574,000,000	$ 304,200,000
Number of properties | item			75	47