Investments in Real Estate (Details) (USD $)	3 Months Ended
	Mar. 31, 2015 item sqft	Mar. 31, 2014
Real estate properties
Leasable square feet	71,600,000
Number of investments in properties that caused any tenant to be 10% of more of total assets	0
Mortgages acquired		$ 45,900,000
Contingent consideration associated with acquisition	0	0
Total revenue	246,867,000	221,572,000
Income from continuing operations	67,581,000	54,916,000
In-place lease, depreciation and amortization to expense	21,600,000	20,100,000
Net (increase) decrease to rental income for capitalized above market and below market leases	1,700,000	2,000,000
Net increase (decrease) to rental revenue
2015	(5,889,000)
2016	(7,864,000)
2017	(7,809,000)
2018	(7,549,000)
2019	(6,563,000)
Thereafter	29,291,000
Totals	(6,383,000)
Increase to amortization expense
2015	65,083,000
2016	86,533,000
2017	85,333,000
2018	82,889,000
2019	72,831,000
Thereafter	429,602,000
Totals	822,271,000
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	209,900,000	656,700,000
Number of properties	83	337
Initial weighted average contractual lease rate of properties acquired (as a percent)	6.90%	7.00%
Number of states in which properties are located	24	35
Leasable square feet	1,600,000	4,500,000
Leased area (as a percent)	100.00%	100.00%
Weighted average lease term	15 years 6 months	14 years 2 months 12 days
Number of industries in which tenants operate	12	15
Acquisition transaction costs	94,000	454,000
New properties and properties under development | Retail properties
Real estate properties
Property type- percentage of properties acquired	74.20%	87.60%
New properties and properties under development | Office properties
Real estate properties
Property type- percentage of properties acquired		4.60%
New properties and properties under development | Industrial and distribution properties
Real estate properties
Property type- percentage of properties acquired	25.80%	7.80%
Properties under development or expansion
Real estate properties
Number of properties	25	13
Initial weighted average contractual lease rate of properties acquired (as a percent)	9.00%	8.40%
Investments in properties	15,300,000	13,400,000
Aggregate acquisitions
Real estate properties
Real estate acquired	209,900,000	656,700,000
Discounts/(Premiums) recorded upon assumption of mortgages		2,800,000
Land recorded related to acquisition	39,800,000	132,200,000
Buildings and improvements recorded	121,200,000	455,300,000
Intangible assets related to leases recorded	41,600,000	74,000,000
Other assets recorded	14,100,000	901,000
Intangible liabilities related to leases and other assumed liabilities recorded	6,800,000	8,500,000
Mortgages acquired		45,900,000
Total revenue	1,500,000	7,900,000
Income from continuing operations	712,000	3,200,000
Amount of investment allocated by estimate	197,700,000
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	1,300,000	1,400,000
Re-leasing costs included in capitalized costs on existing properties	313,000	192,000
Building and tenant improvements included in capitalized costs on existing properties	1,000,000	1,200,000
Real estate investments with existing leases
Real estate properties
Real estate acquired	$ 114,200,000	$ 403,800,000
Number of properties	14	90