Basis of Presentation and Significant Accounting Policies - Narrative (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($) property investment	Dec. 31, 2020 USD ($) property segment variable_interest_entity investment property_partnership	Dec. 31, 2019 USD ($) property property_partnership variable_interest_entity investment	Dec. 31, 2018 USD ($)
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of VIEs | variable_interest_entity		2	4
Assets	$ 8,900,094,000	$ 10,000,708,000	$ 8,900,094,000
Liabilities	4,329,236,000	4,723,387,000	4,329,236,000
Noncontrolling interest in consolidated subsidiary	$ 195,431,000	$ 197,503,000	$ 195,431,000
Number of equity method investments | investment	0	0	0
Number of properties classified as held for sale | property	0	0	0
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 | property		8
Number of development projects | property		1
Number of future development projects | property		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		$ 244,800,000	211,900,000	$ 198,600,000
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	$ 0	$ 74,900,000	$ 0
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 | property_partnership			2
Number of buildings | property_partnership		2
Section 1031 Exchange [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of VIEs | property	2
Finance Partnership [Member] | 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%
Variable Interest Entity, Primary Beneficiary [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Assets	$ 676,700,000	$ 469,300,000	$ 676,700,000
Liabilities	40,100,000	33,900,000	40,100,000
Variable Interest Entity, Primary Beneficiary [Member] | 100 First LLC and 303 Second LLC [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Noncontrolling interest in consolidated subsidiary	189,600,000	191,900,000	189,600,000
Variable Interest Entity, Primary Beneficiary [Member] | Real estate investment [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Assets	$ 598,000,000.0	$ 394,600,000	$ 598,000,000.0