Financial Instruments and Fair Value Measurements - Schedule of Derivative Financial Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Fair value, liability derivatives	$ (17,374)	$ (7,304)
Fair value, asset derivatives	3,528	7,018
Derivative financial instruments (Note 8)	3,528	7,018
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	100,000
Core Portfolio
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	531,885
Fair value, derivatives, net	$ (11,975)	(310)
Core Portfolio | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2020-07
Fair value, liability derivatives	$ (6,100)	(2,900)
Core Portfolio | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	286,058
Fair value, liability derivatives	(15,104)	(6,332)
Core Portfolio | Interest Rate Swaps | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	245,827
Fair value, asset derivatives	3,129	6,022
Derivative financial instruments (Note 8)	$ 3,129	6,022
Core Portfolio | Interest Rate Swaps | Minimum | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2012-12
Derivative Maturity Date	2022-03
Strike Rate	2.22%
Core Portfolio | Interest Rate Swaps | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2013-02
Derivative Maturity Date	2019-11
Strike Rate	1.24%
Core Portfolio | Interest Rate Swaps | Maximum | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2020-07
Derivative Maturity Date	2030-07
Strike Rate	3.77%
Core Portfolio | Interest Rate Swaps | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2017-12
Derivative Maturity Date	2026-06
Strike Rate	3.77%
Fund Portfolio | Fund II
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 42,564
Fair value, derivatives, net	33	108
Fund Portfolio | Fund II | Interest Rate Swaps | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 19,264
Derivative Effective Date	2014-10
Derivative Maturity Date	2021-11
Fair value, asset derivatives	$ 16	108
Derivative financial instruments (Note 8)	$ 16	108
Fund Portfolio | Fund II | Interest Rate Swaps | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	2.88%
Fund Portfolio | Fund II | Interest Rate Swaps | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	2.88%
Fund Portfolio | Fund II | Interest Rate Cap | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 23,300
Derivative Effective Date	2019-03
Derivative Maturity Date	2022-03
Fair value, asset derivatives	$ 17	0
Derivative financial instruments (Note 8)	$ 17	0
Fund Portfolio | Fund II | Interest Rate Cap | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	3.50%
Fund Portfolio | Fund II | Interest Rate Cap | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	3.50%
Fund Portfolio | Fund IV
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 182,748
Fair value, derivatives, net	329	859
Fund Portfolio | Fund IV | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 2,246
Derivative Effective Date	2019-01
Derivative Maturity Date	2022-04
Fair value, liability derivatives	$ (37)	0
Fund Portfolio | Fund IV | Interest Rate Swaps | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	71,602
Fair value, asset derivatives	366	851
Derivative financial instruments (Note 8)	$ 366	851
Fund Portfolio | Fund IV | Interest Rate Swaps | Minimum | Other Liabilities
Derivatives, Fair Value [Line Items]
Strike Rate	2.61%
Fund Portfolio | Fund IV | Interest Rate Swaps | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2017-03
Derivative Maturity Date	2020-03
Strike Rate	1.82%
Fund Portfolio | Fund IV | Interest Rate Swaps | Maximum | Other Liabilities
Derivatives, Fair Value [Line Items]
Strike Rate	2.61%
Fund Portfolio | Fund IV | Interest Rate Swaps | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2017-11
Derivative Maturity Date	2022-12
Strike Rate	2.11%
Fund Portfolio | Fund IV | Interest Rate Cap | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 108,900
Fair value, asset derivatives	0	8
Derivative financial instruments (Note 8)	$ 0	8
Fund Portfolio | Fund IV | Interest Rate Cap | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2016-07
Derivative Maturity Date	2019-08
Strike Rate	3.00%
Fund Portfolio | Fund IV | Interest Rate Cap | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2016-11
Derivative Maturity Date	2019-12
Strike Rate	3.00%
Fund Portfolio | Fund III | Interest Rate Cap | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 58,000
Derivative Effective Date	2016-12
Derivative Maturity Date	2020-01
Fair value, asset derivatives	$ 0	8
Derivative financial instruments (Note 8)	$ 0	8
Fund Portfolio | Fund III | Interest Rate Cap | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund Portfolio | Fund III | Interest Rate Cap | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund Portfolio | Fund V
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 156,900
Fair value, derivatives, net	(2,233)	(951)
Fund Portfolio | Fund V | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	0
Fair value, asset derivatives	0	21
Derivative financial instruments (Note 8)	0	21
Fund Portfolio | Fund V | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	156,900
Fair value, liability derivatives	$ (2,233)	$ (972)
Fund Portfolio | Fund V | Interest Rate Swaps | Minimum | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2018-01
Derivative Maturity Date	2021-02
Strike Rate	2.27%
Fund Portfolio | Fund V | Interest Rate Swaps | Maximum | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2019-03
Derivative Maturity Date	2024-03
Strike Rate	2.88%