Financial Instruments and Fair Value Measurements - Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair value, liability derivatives	$ (1,467)	$ (3,590)
Fair value, asset derivatives	4,402	2,921
Core Portfolio
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	397,607
Fair value, derivatives, net	2,638	(609)
Core Portfolio \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	149,036
Fair value, liability derivatives	$ (1,438)	(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	$ 248,571
Fair value, asset derivatives	$ 4,076	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%
Fund Portfolio \| Fund II
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 49,060
Fair value, derivatives, net	(29)	(228)
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	19,560
Fair value, liability derivatives	$ (29)	(228)
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Liabilities \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	2.88%
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Liabilities \| Maximum
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	$ 29,500
Fair value, asset derivatives	$ 0	0
Fund Portfolio \| Fund II \| Interest Rate Cap \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	4.00%
Fund Portfolio \| Fund II \| Interest Rate Cap \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	4.00%
Fund Portfolio \| Fund III \| Interest Rate Cap \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 58,000
Fair value, asset derivatives	$ 14	127
Fund Portfolio \| Fund III \| Interest Rate Cap \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund Portfolio \| Fund III \| Interest Rate Cap \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund Portfolio \| Fund IV
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 195,751
Fair value, derivatives, net	312	41
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	0
Fair value, liability derivatives	$ 0	(144)
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Liabilities \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	1.78%
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Liabilities \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	2.11%
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 86,851
Fair value, liability derivatives	$ 295	0
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	1.78%
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
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	$ 17	$ 185
Fund Portfolio \| Fund IV \| Interest Rate Cap \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%
Fund Portfolio \| Fund IV \| Interest Rate Cap \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	3.00%

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

+ References

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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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