Stockholders' Equity of the Company (Details) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended		3 Months Ended		12 Months Ended
	Jan. 13, 2017	Dec. 30, 2016	Dec. 13, 2016	Jan. 31, 2017	Jul. 31, 2015	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 15, 2017	Mar. 30, 2017
Dividends Payable [Line Items]
Aggregate gross proceeds				$ 308,800				$ 326,058	$ 31,117	$ 387,398
Aggregate net proceeds after selling commissions					$ 249,600
Accrued dividend and distributions
Accrued dividends and distributions							$ 222,306	$ 43,448	$ 222,306
Dividends declared per common share		$ 1.90	$ 1.90			$ 0.375	$ 0.375	$ 1.65	$ 3.375	$ 1.40
Payments of special cash dividends	$ 184,300
Payments of dividends, common stock	$ 36,400							$ 340,697	$ 137,444	$ 126,839
Summary of shares and units
Common stock							93,219,439	98,620,333	93,219,439
Common stockholders and noncontrolling unitholders [Member]
Accrued dividend and distributions
Accrued dividends and distributions							$ 220,650	$ 43,448	$ 220,650
Common stock [Member]
Accrued dividend and distributions
Accrued dividends and distributions							$ 212,074	$ 41,914	$ 212,074
Dividends declared per common share							$ 2.275	$ 1.650	$ 3.375	$ 1.400
Summary of shares and units
Common stock							93,219,439	98,620,333	93,219,439
Capital units [Member]
Accrued dividend and distributions
Accrued dividends and distributions							$ 5,418	$ 883	$ 5,418
Preferred Stock [Member]
Accrued dividend and distributions
Accrued dividends and distributions							$ 1,656	$ 0	$ 1,656
Series G Preferred Stock [Member]
Summary of shares and units
Preferred units, issued							4,000,000	0	4,000,000
Preferred stock (in shares)							4,000,000		4,000,000			4,000,000
Series H Preferred Stock [Member]
Summary of shares and units
Preferred units, issued							4,000,000	0	4,000,000
Preferred stock (in shares)							4,000,000		4,000,000		4,000,000
Restricted Stock Units (RSUs) [Member]
Accrued dividend and distributions
Accrued dividends and distributions							$ 3,158	$ 651	$ 3,158
Time-Based Restricted Stock Units (RSUs) [Member]
Summary of shares and units
RSUs							1,395,189	1,488,724	1,395,189
Issuance of Equity under ATM Program [Member]
Dividends Payable [Line Items]
Shares of common stock sold during the period								235,077	451,398	1,866,267
Aggregate gross proceeds								$ 17,700	$ 32,300	$ 140,100
Aggregate net proceeds after selling commissions								17,500	31,900	138,200
Kilroy Realty, L.P. [Member]
Dividends Payable [Line Items]
Aggregate gross proceeds								326,058	31,117	387,398
Accrued dividend and distributions
Accrued dividends and distributions							$ 222,306	43,448	222,306
Payments of dividends, common stock								$ 340,697	$ 137,444	$ 126,839
Kilroy Realty, L.P. [Member] | Capital units [Member]
Summary of shares and units
Noncontrolling common units							2,381,543	2,077,193	2,381,543
Kilroy Realty, L.P. [Member] | Restricted Stock Units (RSUs) [Member]
Accrued dividend and distributions
Accrued dividends and distributions							$ 3,158		$ 3,158
Kilroy Realty, L.P. [Member] | Market measure-based Restricted Stock Units (RSUs) [Member]
Summary of shares and units
Number of RSUs outstanding							659,051	665,110	659,051