Partners' Capital of the Operating Partnership - At-The-Market Stock Offering, Common Units and Accrued Distributions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Aggregate net proceeds after selling commissions	$ 721,665	$ 353,722	$ 130,693
Accrued distributions
Accrued dividends and distributions	59,431	53,219
Common units [Member]
Accrued distributions
Accrued dividends and distributions	$ 575	981
Issuance of Equity under ATM Program [Member]
Class of Stock [Line Items]
Shares of common stock contributed by the Company (in shares)	3,147,110		1,817,195
Common units exchanged for shares of common stock by the Company (in units)	3,147,110		1,817,195
Aggregate gross proceeds	$ 252,000		$ 133,800
Aggregate net proceeds after selling commissions	247,300		132,100
Kilroy Realty L.P. [Member]
Class of Stock [Line Items]
Aggregate net proceeds after selling commissions	721,665	353,722	$ 130,693
Accrued distributions
Accrued dividends and distributions	59,431	53,219
Kilroy Realty L.P. [Member] | Restricted Stock Units (RSUs) [Member]
Accrued distributions
Accrued dividends and distributions	$ 838	$ 820
RSUs (in shares)	1,638,026	1,651,905
Kilroy Realty L.P. [Member] | Market measure-based Restricted Stock Units (RSUs) [Member]
Accrued distributions
Number of RSUs outstanding (in shares)	873,709	932,675
Kilroy Realty L.P. [Member] | Partners Capital Common Unit [Member]
Accrued distributions
Accrued dividends and distributions	$ 59,431	$ 53,219
Kilroy Realty L.P. [Member] | Partners capital general partner [Member]
Accrued distributions
Accrued dividends and distributions	58,018	51,418
Kilroy Realty L.P. [Member] | Partners capital limited partner [Member]
Accrued distributions
Accrued dividends and distributions	$ 575	$ 981
Kilroy Realty L.P. [Member] | Common units [Member]
Summary of units
Company owned common units in the Operating Partnership (in units)	116,035,827	106,016,287
Noncontrolling common units (in units)	1,150,574	2,023,287
Operating Partnership [Member]
Summary of units
Company owned general partnership interest	99.00%	98.10%
Operating Partnership [Member] | Non-Affiliated Investors [Member]
Summary of units
Ownership interest of noncontrolling interest	1.00%	1.90%