Investments in Real Estate (Details) ft² in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) ft²	Jun. 30, 2014 USD ($) ft²	Jun. 30, 2015 USD ($) ft² item	Jun. 30, 2014 USD ($) ft² item
Real estate properties
Leasable square feet | ft²	74.1		74.1
Premiums recorded upon acquisition of mortgages				$ 718,000
Number of investments in properties that caused any tenant to be 10% of more of total assets | item			0
Mortgages acquired				159,700,000
Contingent consideration associated with acquisition	$ 0	$ 0	$ 0	0
Total revenue	253,860,000	228,646,000	500,728,000	450,218,000
Income from continuing operations	66,350,000	$ 62,221,000	133,931,000	117,138,000
In-place lease, depreciation and amortization to expense			43,500,000	40,800,000
Net (increase) decrease to rental income for capitalized above market and below market leases			3,700,000	4,200,000
Net increase (decrease) to rental revenue
2015	(3,949,000)		(3,949,000)
2016	(7,895,000)		(7,895,000)
2017	(7,839,000)		(7,839,000)
2018	(7,579,000)		(7,579,000)
2019	(6,593,000)		(6,593,000)
Thereafter	34,357,000		34,357,000
Totals	502,000		502,000
Increase to amortization expense
2015	43,978,000		43,978,000
2016	87,849,000		87,849,000
2017	86,660,000		86,660,000
2018	84,215,000		84,215,000
2019	74,157,000		74,157,000
Thereafter	437,410,000		437,410,000
Totals	$ 814,269,000		$ 814,269,000
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			$ 931,200,000	$ 1,060,000,000
Number of properties | item			166	402
Initial weighted average contractual lease rate of properties acquired (as a percent)			6.40%	7.10%
Number of states in which properties are located | item			35	39
Leasable square feet | ft²	4.2	6.9	4.2	6.9
Leased area (as a percent)	100.00%	100.00%	100.00%	100.00%
Weighted average lease term			17 years 6 months	12 years 9 months 18 days
Number of industries in which tenants operate | item			16	24
Acquisition transaction costs			$ 298,000	$ 645,000
New properties and properties under development | Retail properties
Real estate properties
Property type- percentage of properties acquired			92.10%	83.00%
New properties and properties under development | Office properties
Real estate properties
Property type- percentage of properties acquired				8.50%
New properties and properties under development | Industrial and distribution properties
Real estate properties
Property type- percentage of properties acquired			7.90%	8.50%
Properties under development or expansion
Real estate properties
Number of properties | item			30	21
Initial weighted average contractual lease rate of properties acquired (as a percent)			9.30%	8.50%
Investments in properties			$ 30,400,000	$ 35,600,000
Aggregate acquisitions
Real estate properties
Real estate acquired			931,200,000	1,060,000,000
Premiums recorded upon acquisition of mortgages				718,000
Land recorded related to acquisition			184,200,000	209,200,000
Buildings and improvements recorded			697,400,000	721,400,000
Intangible assets related to leases recorded			64,000,000	161,800,000
Other assets recorded			5,800,000	901,000
Intangible liabilities related to leases and other assumed liabilities recorded			20,200,000	30,900,000
Mortgages acquired				159,700,000
Total revenue			10,000,000	24,400,000
Income from continuing operations			4,900,000	9,400,000
Amount of investment allocated by estimate			720,000,000
Amount of investment previously allocated by estimate finalized during the period			147,100,000
Investments in Existing Properties
Real estate properties
Total capitalized costs on existing properties	$ 2,500,000	$ 2,700,000	2,500,000	2,700,000
Re-leasing costs included in capitalized costs on existing properties			461,000	467,000
Building improvements included in capitalized costs on existing properties			2,000,000	2,300,000
Real estate investments with existing leases
Real estate properties
Real estate acquired			$ 209,800,000	$ 789,800,000
Number of properties | item			34	149