Financial Instruments and Fair Value Measurements - Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair value, liability derivatives	$ (3,468)	$ (3,590)
Fair value, asset derivatives	2,661	2,921
Fund II
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	49,116
Fair value, derivatives, net	(168)	(228)
Fund II \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	19,616
Fair value, liability derivatives	$ (168)	(228)
Fund II \| Interest Rate Swaps \| Other Liabilities \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	2.88%
Fund II \| Interest Rate Swaps \| Other Liabilities \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	2.88%
Fund II \| Interest Rate Cap \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 29,500
Fair value, asset derivatives	$ 0	0
Fund II \| Interest Rate Cap \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	4.00%
Fund II \| Interest Rate Cap \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	4.00%
Fund III \| Interest Rate Cap \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 58,000
Fair value, asset derivatives	$ 17	127
Fund III \| Interest Rate Cap \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund III \| Interest Rate Cap \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund IV
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 190,056
Fair value, derivatives, net	(342)	41
Fund IV \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	81,156
Fair value, liability derivatives	$ (364)	(144)
Fund IV \| Interest Rate Swaps \| Other Liabilities \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	1.78%
Fund IV \| Interest Rate Swaps \| Other Liabilities \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	1.98%
Fund IV \| Interest Rate Cap \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 108,900
Fair value, asset derivatives	$ 22	185
Fund IV \| Interest Rate Cap \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund IV \| Interest Rate Cap \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Core Portfolio
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 354,033
Fair value, derivatives, net	(314)	(609)
Core Portfolio \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	149,440
Fair value, liability derivatives	$ (2,936)	(3,218)
Core Portfolio \| Interest Rate Swaps \| Other Liabilities \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	1.38%
Core Portfolio \| Interest Rate Swaps \| Other Liabilities \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	3.77%
Core Portfolio \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 204,593
Fair value, asset derivatives	$ 2,622	$ 2,609
Core Portfolio \| Interest Rate Swaps \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	1.24%
Core Portfolio \| Interest Rate Swaps \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	3.77%

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

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-Publisher FASB
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition

Variable interest rate in effect as of the balance sheet date related to the interest rate derivative.

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+ References

No definition available.

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