Stockholders' Equity of the Company - At-The-Market Offering and Derivatives and Distributions (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Dividends Payable [Line Items]
Aggregate net proceeds after selling commissions	$ 721,665	$ 353,722	$ 130,693
Accrued dividend and distributions
Accrued dividends and distributions	$ 59,431	$ 53,219
Summary of shares and units
Common stock	116,035,827	106,016,287
Restricted Stock Units (RSUs) [Member]
Summary of shares and units
RSUs	1,638,026	1,651,905
Common stockholders and noncontrolling unitholders [Member]
Accrued dividend and distributions
Accrued dividends and distributions	$ 59,431	$ 53,219
Common Stock [Member]
Accrued dividend and distributions
Accrued dividends and distributions	$ 58,018	$ 51,418
Summary of shares and units
Common stock	116,035,827	106,016,287
Common units [Member]
Accrued dividend and distributions
Accrued dividends and distributions	$ 575	$ 981
Issuance of Equity under ATM Program [Member]
Dividends Payable [Line Items]
Shares of common stock sold during the period (in shares)	3,147,110		1,817,195
Weighted average price per share of common stock (in dollars per share)	$ 80.08		$ 73.64
Aggregate gross proceeds	$ 252,000		$ 133,800
Aggregate net proceeds after selling commissions	247,300		132,100
Kilroy Realty L.P. [Member]
Dividends Payable [Line Items]
Aggregate net proceeds after selling commissions	721,665	353,722	$ 130,693
Accrued dividend and distributions
Accrued dividends and distributions	59,431	53,219
Kilroy Realty L.P. [Member] | Restricted Stock Units (RSUs) [Member]
Accrued dividend and distributions
Accrued dividends and distributions	$ 838	$ 820
Kilroy Realty L.P. [Member] | Market measure-based Restricted Stock Units (RSUs) [Member]
Summary of shares and units
Number of RSUs outstanding (in shares)	873,709	932,675
Kilroy Realty L.P. [Member] | Common units [Member]
Summary of shares and units
Noncontrolling common units (in units)	1,150,574	2,023,287