Loans Payable - Schedule of outstanding loans payable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Total Principal Outstanding	$ 2,624.3	$ 2,338.0
Fair value adjustment	(18.7)	27.0
Total Loans Payable	$ 2,605.6	2,365.0
Red Canyon at Palomino Park
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	5.34%
Annual interest rate and payment frequency	5.34% paid monthly
Principal amounts outstanding	$ 27.1	27.1
Maturity	Aug. 01, 2020
Green River at Palomino Park
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	5.34%
Annual interest rate and payment frequency	5.34% paid monthly
Principal amounts outstanding	$ 33.2	33.2
Maturity	Aug. 01, 2020
Blue Ridge at Palomino Park
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	5.34%
Annual interest rate and payment frequency	5.34% paid monthly
Principal amounts outstanding	$ 33.4	33.4
Maturity	Aug. 01, 2020
Ashford Meadows Apartments
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	5.17%
Annual interest rate and payment frequency	5.17% paid monthly
Principal amounts outstanding	$ 44.6	44.6
Maturity	Aug. 01, 2020
The Knoll
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.98%
Annual interest rate and payment frequency	3.98% paid monthly
Principal amounts outstanding	$ 16.2	16.4
Maturity	Dec. 05, 2020
Ascent at Windward
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.51%
Annual interest rate and payment frequency	3.51% paid monthly
Principal amounts outstanding	$ 34.6	34.6
Maturity	Jan. 01, 2022
The Palatine
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	4.25%
Annual interest rate and payment frequency	4.25% paid monthly
Principal amounts outstanding	$ 75.5	75.9
Maturity	Jan. 10, 2022
The Forum at Carlsbad
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	4.25%
Annual interest rate and payment frequency	4.25% paid monthly
Principal amounts outstanding	$ 85.3	85.7
Maturity	Mar. 01, 2022
Fusion 1560
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.42%
Annual interest rate and payment frequency	3.42% paid monthly
Principal amounts outstanding	$ 37.4	37.4
Maturity	Jun. 10, 2022
San Diego Office Portfolio
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.62%
Annual interest rate and payment frequency	3.62% paid monthly
Principal amounts outstanding, loans payable	$ 49.0	48.2
Maturity, loans payable	Aug. 15, 2022
The Colorado
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.69%
Annual interest rate and payment frequency	3.69% paid monthly
Principal amounts outstanding	$ 87.7	88.1
Maturity	Nov. 01, 2022
The Legacy at Westwood
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.69%
Annual interest rate and payment frequency	3.69% paid monthly
Principal amounts outstanding	$ 44.7	44.9
Maturity	Nov. 01, 2022
Regents Court
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.69%
Annual interest rate and payment frequency	3.69% paid monthly
Principal amounts outstanding	$ 37.9	38.1
Maturity	Nov. 01, 2022
1001 Pennsylvania Avenue
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.70%
Annual interest rate and payment frequency	3.70% paid monthly
Principal amounts outstanding	$ 319.1	320.7
Maturity	Jun. 01, 2023
Biltmore at Midtown
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.94%
Annual interest rate and payment frequency	3.94% paid monthly
Principal amounts outstanding	$ 36.4	36.4
Maturity	Jul. 05, 2023
Cherry Knoll
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.78%
Annual interest rate and payment frequency	3.78% paid monthly
Principal amounts outstanding	$ 35.3	35.3
Maturity	Jul. 05, 2023
Lofts at SoDo
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.94%
Annual interest rate and payment frequency	3.94% paid monthly
Principal amounts outstanding	$ 35.1	35.1
Maturity	Jul. 05, 2023
Pacific City
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate and payment frequency	2.00% + LIBOR paid monthly
Principal amounts outstanding	$ 105.0	0.0
Maturity	Oct. 01, 2023
Birkdale Village
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	4.30%
Annual interest rate and payment frequency	4.30% paid monthly
Principal amounts outstanding	$ 78.3	0.0
Maturity	Apr. 01, 2024
1401 H Street, NW
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.65%
Annual interest rate and payment frequency	3.65% paid monthly
Principal amounts outstanding	$ 115.0	115.0
Maturity	Nov. 05, 2024
The District at La Frontera | Loan A
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.84%
Annual interest rate and payment frequency	3.84% paid monthly
Principal amounts outstanding	$ 39.1	39.3
Maturity	Dec. 01, 2024
The District at La Frontera | Loan B
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	4.96%
Annual interest rate and payment frequency	4.96% paid monthly
Principal amounts outstanding	$ 4.3	4.4
Maturity	Dec. 01, 2024
Circa Green Lake
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.71%
Annual interest rate and payment frequency	3.71% paid monthly
Principal amounts outstanding	$ 52.0	52.0
Maturity	Mar. 05, 2025
Union - South Lake Union
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.66%
Annual interest rate and payment frequency	3.66% paid monthly
Principal amounts outstanding	$ 57.0	57.0
Maturity	Mar. 05, 2025
Holly Street Village
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.65%
Annual interest rate and payment frequency	3.65% paid monthly
Principal amounts outstanding	$ 81.0	81.0
Maturity	May 01, 2025
Henley at Kingstowne
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.60%
Annual interest rate and payment frequency	3.60% paid monthly
Principal amounts outstanding	$ 71.0	71.0
Maturity	May 01, 2025
32 South State Street
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	4.48%
Annual interest rate and payment frequency	4.48% paid monthly
Principal amounts outstanding	$ 24.0	24.0
Maturity	Jun. 06, 2025
Vista Station Office Portfolio | Loan A
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	4.00%
Annual interest rate and payment frequency	4.00% paid monthly
Principal amounts outstanding	$ 20.4	20.5
Maturity	Jul. 01, 2025
Vista Station Office Portfolio | Loan B
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	4.20%
Annual interest rate and payment frequency	4.20% paid monthly
Principal amounts outstanding	$ 44.5	44.7
Maturity	Nov. 01, 2025
780 Third Avenue | Loan A
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.55%
Annual interest rate and payment frequency	3.55% paid monthly
Principal amounts outstanding	$ 150.0	150.0
Maturity	Aug. 01, 2025
780 Third Avenue | Loan B
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.55%
Annual interest rate and payment frequency	3.55% paid monthly
Principal amounts outstanding	$ 20.0	20.0
Maturity	Aug. 01, 2025
701 Brickell Avenue
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.66%
Annual interest rate and payment frequency	3.66% paid monthly
Principal amounts outstanding	$ 184.0	184.0
Maturity	Apr. 01, 2026
Marketplace at Mill Creek
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.82%
Annual interest rate and payment frequency	3.82% paid monthly
Principal amounts outstanding	$ 39.6	0.0
Maturity	Sep. 11, 2027
Overlook At King Of Prussia
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.82%
Annual interest rate and payment frequency	3.82% paid monthly
Principal amounts outstanding	$ 40.8	0.0
Maturity	Sep. 11, 2027
Winslow Bay
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.82%
Annual interest rate and payment frequency	3.82% paid monthly
Principal amounts outstanding	$ 25.8	0.0
Maturity	Sep. 11, 2027
1900 K Street, NW
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.93%
Annual interest rate and payment frequency	3.93% paid monthly
Principal amounts outstanding	$ 163.0	163.0
Maturity	Apr. 01, 2028
99 High Street
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Annual interest rate	3.90%
Annual interest rate and payment frequency	3.90% paid monthly
Principal amounts outstanding	$ 277.0	$ 277.0
Maturity	Mar. 01, 2030
LIBOR | Pacific City
Loans Payable - Schedule of outstanding mortgage loans payable secured by properties [Line Items]
Basis spread on variable rate	2.00%