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, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Aggregate net proceeds after selling commissions	$ 353,722	$ 130,693	$ 326,058
Summary of units
Ownership interest of noncontrolling interest	1.90%	2.00%
Accrued distributions
Accrued dividends and distributions	$ 53,219	$ 47,559
Common units [Member]
Accrued distributions
Accrued dividends and distributions	981	$ 922
Issuance of Equity under ATM Program [Member]
Class of Stock [Line Items]
Shares of common stock contributed by the Company (in shares)		1,817,195	235,077
Common units exchanged for shares of common stock by the Company (in units)		1,817,195	235,077
Aggregate gross proceeds		$ 133,800	$ 17,700
Aggregate net proceeds after selling commissions		132,100	17,500
Kilroy Realty L.P. [Member]
Class of Stock [Line Items]
Aggregate net proceeds after selling commissions	353,722	130,693	$ 326,058
Accrued distributions
Accrued dividends and distributions	53,219	47,559
Kilroy Realty L.P. [Member] | Restricted Stock Units (RSUs) [Member]
Accrued distributions
Accrued dividends and distributions	$ 820	$ 797
RSUs (in shares)	1,651,905	1,711,628
Kilroy Realty L.P. [Member] | Market measure-based Restricted Stock Units (RSUs) [Member]
Accrued distributions
Number of RSUs outstanding (in shares)	932,675	1,018,337
Kilroy Realty L.P. [Member] | Partners Capital Common Unit [Member]
Accrued distributions
Accrued dividends and distributions	$ 53,219	$ 47,559
Kilroy Realty L.P. [Member] | Partners capital general partner [Member]
Accrued distributions
Accrued dividends and distributions	51,418	45,840
Kilroy Realty L.P. [Member] | Partners capital limited partner [Member]
Accrued distributions
Accrued dividends and distributions	$ 981	$ 922
Kilroy Realty L.P. [Member] | Common units [Member]
Summary of units
Company owned common units in the Operating Partnership (in units)	106,016,287	100,746,988
Noncontrolling common units (in units)	2,023,287	2,025,287
Operating Partnership [Member]
Summary of units
Company owned general partnership interest	98.10%	98.00%