Financial Instruments and Fair Value Measurements - Schedule of Derivative Financial Instruments (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Fair value, liability derivatives	$ (37,354)	$ (7,304)
Fair value, asset derivatives	1,057	7,018
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	156,000
Core Portfolio
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	587,574
Fair value, derivatives, net	$ (30,545)	(310)
Core Portfolio \| Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2020-07
Fair value, liability derivatives	$ (10,600)	(2,900)
Core Portfolio \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	424,074
Fair value, liability derivatives	(31,570)	(6,332)
Core Portfolio \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	163,500
Fair value, asset derivatives	$ 1,025	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	1.71%
Core Portfolio \| Interest Rate Swaps \| Minimum \| Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2014-10
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	2016-07
Derivative Maturity Date	2021-06
Strike Rate	1.70%
Fund Portfolio \| Fund II
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 42,501
Fair value, derivatives, net	(141)	108
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 19,201
Derivative Effective Date	2014-10
Derivative Maturity Date	2021-11
Fair value, liability derivatives	$ (150)	0
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Fair value, asset derivatives	$ 0	108
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Minimum \| Other Liabilities
Derivatives, Fair Value [Line Items]
Strike Rate	2.88%
Fund Portfolio \| Fund II \| Interest Rate Swaps \| Maximum \| Other Liabilities
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	$ 9	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	$ 202,114
Fair value, derivatives, net	(855)	859
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	70,114
Fair value, liability derivatives	(878)	0
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 23,100
Derivative Effective Date	2017-03
Derivative Maturity Date	2020-03
Fair value, asset derivatives	$ 23	851
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Minimum \| Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2017-03
Derivative Maturity Date	2022-04
Strike Rate	1.97%
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Minimum \| Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	1.82%
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Maximum \| Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2019-05
Derivative Maturity Date	2022-12
Strike Rate	4.00%
Fund Portfolio \| Fund IV \| Interest Rate Swaps \| Maximum \| Other Assets
Derivatives, Fair Value [Line Items]
Strike Rate	1.82%
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
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
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	(4,756)	(951)
Fund Portfolio \| Fund V \| Interest Rate Swaps \| Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	156,900
Fair value, liability derivatives	(4,756)	(972)
Fund Portfolio \| Fund V \| Interest Rate Swaps \| Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	0
Fair value, asset derivatives	$ 0	$ 21
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%

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- Definition

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

+ References

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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- Definition

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

+ References

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