Investments in Real Estate (Details) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended																	1 Months Ended	9 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2013 sqft	Sep. 30, 2012	Sep. 30, 2013 item sqft	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Pro Forma	Sep. 30, 2012 Pro Forma	Sep. 30, 2013 ARCT item sqft	Sep. 30, 2013 ARCT Retail properties	Sep. 30, 2013 ARCT Office properties	Sep. 30, 2013 ARCT Industrial and distribution properties	Sep. 30, 2013 Maximum	Sep. 30, 2013 New properties and properties under development or expansion item sqft	Sep. 30, 2012 New properties and properties under development or expansion item sqft	Sep. 30, 2013 New properties and properties under development or expansion Retail properties	Sep. 30, 2012 New properties and properties under development or expansion Retail properties	Sep. 30, 2013 New properties and properties under development or expansion Office properties	Sep. 30, 2012 New properties and properties under development or expansion Office properties	Sep. 30, 2013 New properties and properties under development or expansion Industrial and distribution properties	Sep. 30, 2012 New properties and properties under development or expansion Industrial and distribution properties	Sep. 30, 2013 New properties and properties under development or expansion Manufacturing properties	Sep. 30, 2012 New properties and properties under development or expansion Manufacturing properties	Sep. 30, 2013 New properties San Diego, CA	Sep. 30, 2013 Properties under development or expansion item	Dec. 31, 2012 Aggregate acquisitions	Sep. 30, 2013 Aggregate acquisitions	Sep. 30, 2013 Investments in Existing Properties	Sep. 30, 2012 Investments in Existing Properties	Sep. 30, 2013 Real estate investments with existing leases item
Investments in real estate properties
Investments in properties								$ 3,200,000,000															$ 45,400,000	$ 28,800,000
Real estate acquired													1,370,000,000	717,600,000												4,520,000,000			4,250,000,000
Number of properties								515					407	245										19					756
Estimated initial weighted average contractual lease rate of properties acquired (as a percent)													7.00%	7.10%										8.50%
Number of states in which properties are located								44					40	34
Leasable square feet	61,200,000		61,200,000					16,000,000					8,000,000	7,000,000
Leased area (as a percent)								100.00%					100.00%	100.00%
Weighted average lease term								12 years 2 months 12 days					14 years 1 month 6 days	13 years 8 months 12 days
Number of tenants								69
Number of industries in which tenants operate								28					21	20
Property type- percentage of properties acquired									53.50%	14.50%	32.00%				84.10%	77.00%	10.40%	1.20%	3.10%	16.40%	2.40%	5.40%
Acquisition transaction costs								12,900,000					1,700,000	1,400,000
Number of investments in properties that caused any tenant to be 10% of more of total assets			0
Investments to any one tenant as a percentage of total assets												10.00%
Land recorded related to acquisition																										769,400,000
Buildings and improvements recorded																										3,100,000,000
Intangible assets related to leases recorded																										765,300,000
Other assets recorded																										13,500,000
Intangible liabilities related to leases and other assumed liabilities recorded																										143,400,000
Premiums recorded upon assumption of mortgages																										28,400,000
Contingent consideration associated with acquisition	0		0
Total revenue	199,304,000	117,149,000	550,398,000	340,809,000																						148,100,000
Income from continuing operations	45,295,000	33,623,000	130,535,000	107,863,000																						36,000,000
Amount of investment allocated by estimate																										3,430,000,000
Amount of investment previously allocated by estimate finalized during the period																									106,400,000	1,090,000,000
Total capitalized costs on existing properties																											5,900,000	4,500,000
Re-leasing costs included in capitalized costs on existing properties																											1,100,000	1,200,000
Building and tenant improvements included in capitalized costs on existing properties																											4,800,000	3,300,000
Intangible value of the in-place leases acquired	832,583,000		832,583,000		235,914,000																								596,400,000
Intangible value of above-market leases acquired	215,532,000		215,532,000		40,389,000																								177,800,000
Intangible value of below-market leases acquired																													116,000,000
In-place lease, depreciation and amortization to expense			46,400,000	10,300,000
Net (increase) decrease to rental income for capitalized above market and below market leases			6,200,000	1,400,000
Net decrease to rental revenue
2013	(2,419,000)		(2,419,000)
2014	(9,771,000)		(9,771,000)
2015	(9,614,000)		(9,614,000)
2016	(9,469,000)		(9,469,000)
2017	(9,447,000)		(9,447,000)
Thereafter	(20,852,000)		(20,852,000)
Totals	(61,572,000)		(61,572,000)
Increase to amortization expense
2013	19,092,000		19,092,000
2014	75,917,000		75,917,000
2015	72,436,000		72,436,000
2016	70,236,000		70,236,000
2017	69,239,000		69,239,000
Thereafter	449,708,000		449,708,000
Totals	756,628,000		756,628,000
Estimated merger-related costs							12,400,000
Pro Forma Information
Total revenue						606,100,000	540,100,000
Income from continuing operations						$ 166,000,000	$ 146,100,000