Financial Instruments and Fair Value Measurements - Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Fair value, liability derivatives	$ (1,238)	$ (1,467)
Fair value, asset derivatives	9,958	4,402
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	50,000
Core Portfolio
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	406,687
Fair value, derivatives, net	7,220	2,638
Core Portfolio \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	69,864
Fair value, liability derivatives	$ (1,238)	(1,438)
Core Portfolio \| Interest Rate Swaps \| Other Liabilities \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	2.90%
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	$ 336,823
Fair value, asset derivatives	$ 8,458	4,076
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	$ 49,003
Fair value, derivatives, net	155	(29)
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	0
Fair value, liability derivatives	$ 0	(29)
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 Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 19,503
Fair value, asset derivatives	$ 155	0
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	2.88%
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Assets \| 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	$ 48	14
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	$ 195,417
Fair value, derivatives, net	1,287	312
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	86,517
Fair value, asset derivatives	$ 1,222	295
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	$ 65	17
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%
Fund Portfolio \| Fund V \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 16,900
Fair value, asset derivatives	$ 10	$ 0
Fund Portfolio \| Fund V \| Interest Rate Swaps \| Other Assets \| Minimum
Derivatives, Fair Value [Line Items]
Strike Rate	2.41%
Fund Portfolio \| Fund V \| Interest Rate Swaps \| Other Assets \| Maximum
Derivatives, Fair Value [Line Items]
Strike Rate	2.41%

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
-Name Accounting Standards Codification
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+ References

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+ References

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

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