Fair Value Measurements - Schedule of Fair Values of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investments in Property Funds	$ 193,081	$ 183,216
Investments in Alternative Investment Funds	142,464	140,171
Total real estate fund investments	335,545	323,387
Marketable securities	21,524	20,159
Total assets	357,069	343,546
Interest rate swap liabilities	160,473	194,196
Total liabilities	160,473	194,196
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	21,524	20,159
Total assets	21,524	20,159
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Interest rate swap liabilities	160,473	194,196
Total liabilities	160,473	194,196
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investments in Property Funds	193,081	183,216
Investments in Alternative Investment Funds	142,464	140,171
Total real estate fund investments	335,545	323,387
Total assets	$ 335,545	$ 323,387