Basis of Presentation and Significant Accounting Policies - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($) variable_interest_entity	Dec. 31, 2019 USD ($) segment variable_interest_entity property_partnership	Dec. 31, 2018 USD ($) variable_interest_entity property_partnership	Dec. 31, 2017 USD ($)	Dec. 31, 2019 building	Dec. 31, 2019 property	Dec. 31, 2019	Dec. 31, 2019 property_partnership	Dec. 31, 2019 project	Jan. 01, 2019 USD ($) ground_lease
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of VIEs | variable_interest_entity		4	3
VIE asset amount	$ 676,700,000	$ 676,700,000	$ 615,400,000
VIE, carrying amount, liabilities	40,100,000	40,100,000	45,100,000
Noncontrolling interest in consolidated subsidiary	195,431,000	195,431,000	192,363,000
Lease income		826,472,000
Leasing costs		7,615,000
Number of ground leases | ground_lease										4
Weighted average discount rate							5.15%
Right of use ground lease asset (6)	96,348,000	96,348,000								$ 96,272,000
Ground lease liabilities	$ 98,400,000	$ 98,400,000								98,349,000
Number of consolidated property partnerships | property_partnership		3
Conversion ratio		1
Income tax provision		$ 0	0	$ 0
Number of operating segments | segment		1
Washington [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings					1	8
Number of development projects | project									1
Building and building improvements [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Depreciation of real estate		$ 211,900,000	198,600,000	$ 190,500,000
Kilroy Realty Finance, Inc. [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percentage of general partnership interest owned by wholly-owned subsidiary of the Company		1.00%
Minimum [Member] | Building and building improvements [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property depreciable lives		25 years
Minimum [Member] | Leasehold improvements [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property depreciable lives		1 year
Maximum [Member] | Building and building improvements [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property depreciable lives		40 years
Maximum [Member] | Leasehold improvements [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property depreciable lives		20 years
Lease Agreements [Member] | Minimum [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Finite-lived intangible asset, useful life		1 year
Lease Agreements [Member] | Maximum [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Finite-lived intangible asset, useful life		20 years
Disposal group, disposed of by sale [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Restricted cash			$ 113,100,000
100 First LLC and 303 Second LLC [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of VIEs	2		2
Noncontrolling interest in consolidated subsidiary	$ 189,600,000	$ 189,600,000	$ 186,400,000
Real estate investment [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
VIE asset amount	$ 598,000,000.0	598,000,000.0	$ 543,900,000
Accounting Standards Update 2016-02 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Leasing costs		11,400,000
Cumulative adjustment										(3,146,000)
Right of use ground lease asset (6)										82,900,000
Ground lease liabilities										87,400,000
Accounting Standards Update 2016-02 [Member] | Accumulated Distributions in Excess of Net Income [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Cumulative adjustment										$ (3,146,000)
Property expense reimbursement [Member] | Accounting Standards Update 2016-02 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Lease income		$ 13,900,000
Development properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings								3	2
Development properties [Member] | 100 First LLC and 303 Second LLC [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of buildings | property_partnership								2