Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to Kennedy-Wilson Holdings, Inc. common shareholders	$ 65.9	$ (25.1)	$ 275.7	$ (77.1)
Weighted average shares outstanding for basic (in shares)	138,934,754	140,119,442	138,989,733	140,181,438
(Loss) income per share (in dollars per share)	$ 0.48	$ (0.18)	$ 1.98	$ (0.55)
Weighted average shares outstanding (in shares)	139,437,126	140,119,442	140,565,582	140,181,438
Loss (income) per share (in dollars per share)	$ 0.47	$ (0.18)	$ 1.96	$ (0.55)
Potentially dilutive securities (in shares)	14,623,575	13,543,942	13,557,133	13,691,560