Financial Instruments and Fair Value Measurements - Schedule of Derivative Financial Instruments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Fair value, liability derivatives	$ (53,194)	$ (7,304)
Fair value, asset derivatives	1,217	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	587,261
Fair value, derivatives, net	(45,814)	(310)
Core Portfolio | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 155,400	0
Derivative Effective Date	2020-07
Fair value, liability derivatives	$ (15,700)	(2,900)
Core Portfolio | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	423,761
Fair value, liability derivatives	(46,395)	(6,332)
Core Portfolio | Interest Rate Swaps | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	163,500
Fair value, asset derivatives	$ 581	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 | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2019-10
Fair value, liability derivatives	$ (100)
Fund Portfolio | Interest Rate Swaps | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2019-11
Fair value, liability derivatives	$ 600
Fund Portfolio | Fund II
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	42,438
Fair value, derivatives, net	(177)	108
Fund Portfolio | Fund II | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 19,138
Derivative Effective Date	2014-10
Derivative Maturity Date	2021-11
Fair value, liability derivatives	$ (180)	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	$ 3	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	$ 193,001
Fair value, derivatives, net	(1,066)	859
Fund Portfolio | Fund IV | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	65,501
Fair value, liability derivatives	(1,068)	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	$ 2	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	$ 104,400
Fair value, asset derivatives	$ 0	8
Fund Portfolio | Fund IV | Interest Rate Cap | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2016-11
Derivative Maturity Date	2019-12
Strike Rate	3.00%
Fund Portfolio | Fund IV | Interest Rate Cap | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2019-07
Derivative Maturity Date	2021-07
Strike Rate	3.50%
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	(4,920)	(951)
Fund Portfolio | Fund V | Interest Rate Swaps | Other Liabilities
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	156,900
Fair value, liability derivatives	(5,551)	(972)
Fund Portfolio | Fund V | Interest Rate Swaps | Other Assets
Derivatives, Fair Value [Line Items]
Aggregate Notional Amount	$ 0
Derivative Effective Date	2019-11
Fair value, asset derivatives	$ 631	$ 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	1.45%
Fund Portfolio | Fund V | Interest Rate Swaps | Minimum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Maturity Date	2024-09
Strike Rate	1.25%
Fund Portfolio | Fund V | Interest Rate Swaps | Maximum | Other Liabilities
Derivatives, Fair Value [Line Items]
Derivative Effective Date	2019-10
Derivative Maturity Date	2024-10
Strike Rate	2.88%
Fund Portfolio | Fund V | Interest Rate Swaps | Maximum | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Maturity Date	2024-10
Strike Rate	1.28%