Investments in Real Estate Partnerships - Scheduled Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
2017	$ 122,867
2018	91,147
2019	539,702
2020	328,177
2021	582,466
Beyond 5 Years	1,947,384
Unamortized debt discounts (premiums)	(16,766)
Long-term Debt	3,594,977	$ 1,642,420
Mortgages [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Debt	520,193	384,786
Mortgages [Member] \| Scheduled Principal Payments [Member]
Schedule of Equity Method Investments [Line Items]
2017	10,641
2018	9,360
2019	11,122
2020	11,426
2021	11,618
Beyond 5 Years	37,056
Unamortized debt discounts (premiums)	0
Long-term Debt	91,223
Mortgages [Member] \| Mortgage Loan Maturities [Member]
Schedule of Equity Method Investments [Line Items]
2017	112,226
2018	21,787
2019	78,580
2020	66,751
2021	5,848
Beyond 5 Years	260,328
Unamortized debt discounts (premiums)	9,316
Long-term Debt	554,836
Unsecured Debt [Member]
Schedule of Equity Method Investments [Line Items]
Long-term Debt	2,325,656	$ 892,170
Unsecured Debt [Member] \| Unsecured Maturities [Member]
Schedule of Equity Method Investments [Line Items]
2017	0
2018	60,000
2019	450,000
2020	250,000
2021	565,000
Beyond 5 Years	1,650,000
Unamortized debt discounts (premiums)	(26,082)
Long-term Debt	2,948,918
Unconsolidated Investments in Partnership [Member]
Schedule of Equity Method Investments [Line Items]
2017	51,081
2018	93,111
2019	260,548
2020	280,760
2021	203,271
Beyond 5 Years	636,309
Unamortized debt discounts (premiums)	(10,351)
Long-term Debt	1,514,729
Unconsolidated Investments in Partnership [Member] \| Mortgages [Member] \| Scheduled Principal Payments [Member]
Schedule of Equity Method Investments [Line Items]
2017	21,059
2018	19,852
2019	16,823
2020	10,818
2021	7,569
Beyond 5 Years	3,011
Unamortized debt discounts (premiums)	0
Long-term Debt	79,132
Unconsolidated Investments in Partnership [Member] \| Mortgages [Member] \| Mortgage Loan Maturities [Member]
Schedule of Equity Method Investments [Line Items]
2017	30,022
2018	73,259
2019	224,090
2020	269,942
2021	195,702
Beyond 5 Years	633,298
Unamortized debt discounts (premiums)	(10,351)
Long-term Debt	1,415,962
Unconsolidated Investments in Partnership [Member] \| Unsecured Debt [Member] \| Unsecured Maturities [Member]
Schedule of Equity Method Investments [Line Items]
2017	0
2018	0
2019	19,635
2020	0
2021	0
Beyond 5 Years	0
Unamortized debt discounts (premiums)	0
Long-term Debt	19,635
Unconsolidated Investments in Partnership, Pro-Rata Share [Member] [Member]
Schedule of Equity Method Investments [Line Items]
2017	19,647
2018	24,448
2019	91,039
2020	100,402
2021	73,369
Beyond 5 Years	215,071
Unamortized debt discounts (premiums)	(3,365)
Long-term Debt	$ 520,611

Amount, after accumulated amortization, of debt discount (premium).

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