Real Estate Intangibles (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Assets:
Gross historical cost	$ 928,910	$ 673,684
Accumulated amortization	375,767	257,242
Net book value	553,143	416,442
Weighted-average amortization period in years	17 years 7 months	16 years 4 months
Liabilities:
Gross historical cost	86,458	86,526
Accumulated amortization	31,580	27,753
Net book value	54,878	58,773
Weighted-average amortization period in years	14 years 3 months	14 years 3 months
Intangible Liability [Member]
Estimated Aggregate Amortization For Acquired Lease Intangibles Expected To Be Recognized [Line Items]
2013	3,523
2014	6,644
2015	5,659
2016	5,247
2017	4,928
Thereafter	28,877
Totals	54,878
Intangible Asset [Member]
Estimated Aggregate Amortization For Acquired Lease Intangibles Expected To Be Recognized [Line Items]
2013	142,493
2014	151,663
2015	46,355
2016	23,954
2017	23,612
Thereafter	165,066
Totals	553,143
In place lease intangibles [Member]
Assets:
Gross historical cost	797,343	541,729
Above market tenant leases [Member]
Assets:
Gross historical cost	54,238	56,086
Below market ground leases [Member]
Assets:
Gross historical cost	61,461	61,450
Lease commissions [Member]
Assets:
Gross historical cost	15,868	14,419
Below market tenant leases [Member]
Liabilities:
Gross historical cost	76,968	77,036
Above market ground leases [Member]
Liabilities:
Gross historical cost	$ 9,490	$ 9,490