Organization, Ownership and Basis of Presentation (Details Textual) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) VIE	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 building	Sep. 30, 2018 VIE	Sep. 30, 2018 property_partnership	Sep. 30, 2018 project	Sep. 30, 2018 a	Sep. 30, 2018 property	Dec. 31, 2017 USD ($) VIE
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Lease-up properties occupancy percentage			95.00%
Lease-up properties occupancy duration			1 year
Common limited partnership interest in the Operating Partnership	2.00%	2.10%	2.00%	2.10%								2.10%
Number of VIEs | VIE							5					2
Number of entities, increase (decrease) | VIE	3
VIE assets					$ 828,300							$ 426,500
VIE liabilities					86,200							27,300
Noncontrolling interest in VIE					193,675							181,575
Ground leases, future minimum payments receivable					248,600
San Francisco, California [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Buildings | building						1
Office properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Buildings						104					4
Area of land | a										80
Office properties [Member] | Washington [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Buildings | property											12
Development properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Buildings | project									3
Number of property partnerships | property_partnership								3
Development properties [Member] | Washington [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Buildings | property											1
Development sites [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Buildings | project									7
Properties and development projects [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property ownership percentage			100.00%
Real estate investment [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
VIE assets					753,800							382,100
Fixed-price contract [Member] | Parking revenue [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Revenue	$ 6,700	$ 6,600	$ 20,200	$ 20,300
101 First LLC and 303 Second LLC [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Common general partnership interest in the Operating Partnership	56.00%		56.00%
Noncontrolling interest in VIE					$ 187,600							$ 175,400
101 First LLC and 303 Second LLC [Member] | Development properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of property partnerships | property_partnership								2
Redwood LLC [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Buildings | building						2
Common general partnership interest in the Operating Partnership	93.00%		93.00%
Kilroy Realty Finance, Inc. [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Common general partnership interest in the Finance Partnership (percentage)			1.00%
Kilroy Realty L.P. [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percentage of limited partnership interest owned by Operating Partnership			99.00%