Consolidated Statements of Equity (Unaudited) - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Common Stock Additional Paid-in Capital	Common Stock Distributions in Excess of Earnings	Total Stockholders' Equity	Noncontrolling Interest [Member]	Kilroy Realty, L.P. [Member]	Kilroy Realty, L.P. [Member] Noncontrolling Interest [Member]
Beginning Balance at Dec. 31, 2013	$ 2,516,160	$ 192,411	$ 822	$ 2,478,975	$ (210,896)	$ 2,461,312	$ 54,848
Beginning Balance, shares at Dec. 31, 2013			82,153,944
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	152,378				149,367	149,367	3,011	$ 152,378	$ 201
Issuance of common stock, shares (Note 8)			370,700
Issuance of common stock (Note 8)	22,136		$ 4	22,132		22,136
Issuance of share-based compensation awards	1,281			1,281		1,281
Noncash amortization of share-based compensation	10,345			10,345		10,345		10,345
Exercise of stock options (Note 10)			482,000
Exercise of stock options (Note 10)	20,537		$ 4	20,533		20,537
Repurchase of common stock and restricted stock units, shares			(48,017)
Repurchase of common stock and restricted stock units	(2,861)			(2,861)		(2,861)
Settlement of restricted stock units for shares of common stock, shares			108,529
Settlement of restricted stock units for shares of common stock	0			0		0		0
Stock Issued During Period, Shares, in Connection with Early Exchange of Notes			431,270
Stock Issued During Period, Value, in Connection with Early Exchange of Notes	223		$ 4	219		223		223
Stock Received During Period, Call Options			(111,206)
Exchange of common units of the Operating Parternship, shares			1,000
Exchange of common units of the Operating Partnership	0			28		28	(28)
Adjustment for noncontrolling interest	0			(370)		(370)	370
Contribution by noncontrolling interest in consolidated subsidiary	336						336	336	336
Preferred dividends	(9,938)				(9,938)	(9,938)		(9,938)
Dividends declared per common share and common unit ($1.05 per share/unit)	(90,228)				(88,332)	(88,332)	(1,896)
Ending Balance, shares at Sep. 30, 2014			83,388,220
Ending Balance at Sep. 30, 2014	2,620,369	192,411	$ 834	2,530,282	(159,799)	2,563,728	56,641
Beginning Balance at Dec. 31, 2014	2,723,936	192,411	$ 863	2,635,900	(162,964)	2,666,210	57,726
Beginning Balance, shares at Dec. 31, 2014			86,259,684
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	209,296				205,446	205,446	3,850	209,296	211
Issuance of common stock, shares (Note 8)			5,640,033
Issuance of common stock (Note 8)	387,509		$ 56	387,453		387,509
Issuance of share-based compensation awards	1,268			1,268		1,268
Noncash amortization of share-based compensation	13,621			13,621		13,621		13,621
Exercise of stock options (Note 10)			265,000
Exercise of stock options (Note 10)	11,292		$ 3	11,289		11,292
Repurchase of common stock and restricted stock units, shares			(39,317)
Repurchase of common stock and restricted stock units	(3,121)			(3,121)		(3,121)
Settlement of restricted stock units for shares of common stock, shares			78,937
Settlement of restricted stock units for shares of common stock	0			0		0		0
Exchange of common units of the Operating Parternship, shares			16,030
Exchange of common units of the Operating Partnership	0			467		467	(467)
Adjustment for noncontrolling interest	0			(4,547)		(4,547)	4,547
Contribution by noncontrolling interest in consolidated subsidiary	474							474	$ 474
Preferred dividends	(9,938)				(9,938)	(9,938)		$ (9,938)
Dividends declared per common share and common unit ($1.05 per share/unit)	(97,275)				(95,394)	(95,394)	(1,881)
Ending Balance, shares at Sep. 30, 2015			92,220,367
Ending Balance at Sep. 30, 2015	$ 3,237,062	$ 192,411	$ 922	$ 3,042,330	$ (62,850)	$ 3,172,813	$ 64,249