Investments in Real Estate (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended
	Jun. 30, 2013 sqft	Jun. 30, 2012	Jun. 30, 2013 item sqft	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Pro Forma	Jun. 30, 2012 Pro Forma	Jun. 30, 2013 ARCT item sqft	Jun. 30, 2013 Maximum	Jun. 30, 2013 New properties and properties under development or expansion item sqft	Jun. 30, 2012 New properties and properties under development or expansion item sqft	Jun. 30, 2013 Properties under development or expansion item	Jun. 30, 2013 Aggregate acquisitions	Dec. 31, 2012 Aggregate acquisitions	Jun. 30, 2013 Investments in Existing Properties	Jun. 30, 2012 Investments in Existing Properties	Jun. 30, 2013 Real estate investments with existing leases item
Investments in real estate properties
Investments in properties								$ 3,200,000,000		$ 866,500,000	$ 221,500,000	$ 18,800,000	$ 4,000,000,000				$ 3,970,000,000
Number of properties								515		206	156	19					712
Estimated initial weighted average contractual lease rate of properties acquired (as a percent)										7.00%	7.20%	8.60%
Number of states in which properties are located								44		35	29
Leasable square feet	58,300,000		58,300,000					16,000,000		5,100,000	1,400,000
Leased area (as a percent)								100.00%		100.00%	100.00%
Weighted average lease term								12 years 2 months 12 days		13 years 9 months 18 days	14 years 7 months 6 days
Number of tenants								69
Number of industries in which tenants operate								28		17	7
Acquisition transaction costs								12,600,000		818,000	634,000
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													553,400,000
Buildings and improvements recorded													2,780,000,000
Intangible assets recorded													771,300,000
Other assets recorded													13,700,000
Intangible and assumed liabilities recorded													93,500,000
Premiums recorded upon assumption of mortgages													28,400,000
Contingent consideration associated with acquisition	0		0
Total revenue	184,319,000	113,133,000	355,460,000	224,878,000									86,000,000
Income from continuing operations	50,836,000	38,161,000	82,998,000	75,109,000									16,000,000
Amount of investment allocated by estimate													3,900,000,000
Amount of investment previously allocated by estimate finalized during the period													93,000,000	106,400,000
Total capitalized costs on existing properties															3,300,000	2,400,000
Re-leasing costs included in capitalized costs on existing properties															774,000	698,000
Building and tenant improvements included in capitalized costs on existing properties			(3,294,000)	(2,405,000)											2,500,000	1,700,000
Intangible value of the in-place leases acquired	886,587,000		886,587,000		235,914,000												650,400,000
Intangible value of above-market leases acquired	158,629,000		158,629,000		40,389,000												120,900,000
Intangible value of below-market leases acquired																	93,500,000
In-place lease, depreciation and amortization to expense			33,300,000	6,700,000
Net (increase) decrease to rental income for capitalized above market and below market leases			1,400,000	964,000
Net decrease to rental revenue
2013	(1,763,000)		(1,763,000)
2014	(3,602,000)		(3,602,000)
2015	(3,546,000)		(3,546,000)
2016	(3,535,000)		(3,535,000)
2017	(3,500,000)		(3,500,000)
Thereafter	(16,061,000)		(16,061,000)
Totals	(32,007,000)		(32,007,000)
Increase to amortization expense
2013	40,811,000		40,811,000
2014	81,148,000		81,148,000
2015	80,271,000		80,271,000
2016	79,992,000		79,992,000
2017	78,707,000		78,707,000
Thereafter	462,731,000		462,731,000
Totals	823,660,000		823,660,000
Estimated merger-related costs							12,100,000
Pro Forma Information
Total revenue						388,200,000	341,600,000
Income from continuing operations						$ 105,400,000	$ 83,700,000