Partners' Capital of the Operating Partnership - OP Common Units (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, shares issued (in shares)	100,746,988		98,620,333
Net proceeds from issuance of common units (Note 8)	$ 130,744	$ 308,812
Kilroy Realty L.P. [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common units (Note 8)	$ 130,744	$ 308,812
Kilroy Realty L.P. [Member] | Issuance of Equity - at the market offering [Member]
Class of Stock [Line Items]
Common stock, shares issued (in shares)	1,817,195
Common units exchanged for share of common stock by the Company (in units)	1,817,195
At the market stock offering aggregate gross sales price of common units	$ 133,800
Net proceeds from issuance of common units (Note 8)	$ 132,100