Investments in Real Estate (Details) ft² in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) ft²	Sep. 30, 2014 USD ($) ft²	Sep. 30, 2015 USD ($) ft² item	Sep. 30, 2014 USD ($) ft² item
Real estate properties
Leasable square feet | ft²	74.8		74.8
Premiums recorded upon acquisition of mortgages				$ 604,000
Number of investments in properties that caused any tenant to be 10% of more of total assets | item			0
Mortgages acquired				166,700,000
Contingent consideration associated with acquisition	$ 0	$ 0	$ 0	0
Total revenue	258,889,000	235,713,000	759,617,000	685,932,000
Income from continuing operations	67,813,000	$ 73,627,000	201,744,000	190,766,000
In-place lease, depreciation and amortization to expense			65,500,000	62,100,000
Net (Increase) Decrease to Rental Income for Capitalized above Market and below Market Leases			5,800,000	6,400,000
Net increase (decrease) to rental revenue
2015	(2,090,000)		(2,090,000)
2016	(8,353,000)		(8,353,000)
2017	(8,296,000)		(8,296,000)
2018	(8,049,000)		(8,049,000)
2019	(7,059,000)		(7,059,000)
Thereafter	35,278,000		35,278,000
Totals	1,431,000		1,431,000
Increase to amortization expense
2015	22,242,000		22,242,000
2016	88,878,000		88,878,000
2017	87,689,000		87,689,000
2018	85,194,000		85,194,000
2019	75,130,000		75,130,000
Thereafter	444,724,000		444,724,000
Totals	$ 803,857,000		$ 803,857,000
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	$ 1,240,000,000
Number of properties | item			195	439
Initial weighted average contractual lease rate of properties acquired (as a percent)			6.50%	7.10%
Number of states in which properties are located | item			36	42
Leasable square feet | ft²	5.1	8.5	5.1	8.5
Leased area (as a percent)	100.00%	100.00%	100.00%	100.00%
Weighted average lease term			16 years 8 months 12 days	12 years 7 months 6 days
Number of industries in which tenants operate | item			18	27
Acquisition transaction costs			$ 368,000	$ 589,000
New properties and properties under development | Retail properties
Real estate properties
Property type- percentage of properties acquired			87.00%	85.60%
New properties and properties under development | Office properties
Real estate properties
Property type- percentage of properties acquired				7.20%
New properties and properties under development | Industrial and distribution properties
Real estate properties
Property type- percentage of properties acquired			13.00%	7.20%
Properties under development or expansion
Real estate properties
Number of properties | item			30	32
Initial weighted average contractual lease rate of properties acquired (as a percent)			9.90%	8.50%
Investments in properties			$ 37,100,000	$ 69,000,000
Aggregate acquisitions
Real estate properties
Real estate acquired			1,100,000,000	1,240,000,000
Premiums recorded upon acquisition of mortgages				604,000
Land recorded related to acquisition			214,800,000	240,100,000
Buildings and improvements recorded			780,800,000	861,900,000
Intangible assets related to leases recorded			86,600,000	202,000,000
Other assets recorded				901,000
Intangible liabilities related to leases and other assumed liabilities recorded			27,200,000	60,500,000
Mortgages acquired				166,700,000
Total revenue			25,300,000	47,300,000
Income from continuing operations			12,600,000	19,200,000
Amount of investment allocated by estimate			117,200,000
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	$ 5,900,000	$ 4,500,000	5,900,000	4,500,000
Re-leasing costs and real estate improvements included in capitalized costs on existing properties			555,000
Recurring Capital Expenditures Real Estate Improvements			3,800,000
Nonrecurring Building Real Estate Improvements			1,500,000
Real estate investments with existing leases
Real estate properties
Real estate acquired			$ 304,200,000	$ 949,600,000
Number of properties | item			47	180