Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income attributable to Kennedy-Wilson Holdings, Inc. common shareholders	$ 109.6	$ 9.4	$ 107.2	$ 10.2
Dividends allocated to participating securities	(0.8)	(0.2)	(0.8)	(0.5)
Net income attributable to Kennedy-Wilson Holdings, Inc. common shareholders, net of allocation to participating securities	108.8	9.2	106.4	9.7
Dividends declared on common shares	(27.3)	(19.3)	(55.6)	(38.6)
Undistributed earnings (losses) attributable to Kennedy-Wilson Holdings, Inc. common shareholders, net of allocation to participating securities	$ 81.5	$ (10.1)	$ 50.8	$ (28.9)
Distributed earnings per share (in dollars per share)	$ 0.19	$ 0.17	$ 0.38	$ 0.34
Undistributed earnings (losses) per share (in dollars per share)	$ 0.58	$ (0.09)	0.36	(0.25)
Income per basic and diluted share (in dollars per share)			$ 0.74	$ 0.09
Weighted average shares outstanding for basic (in shares)	141,483,445	111,723,952	144,696,059	111,945,354
Weighted average shares outstanding for diluted (in shares)	142,707,159	111,723,952	145,872,617	111,945,354
Dividends declared per common share (in dollars per share)	$ 0.19	$ 0.17	$ 0.38	$ 0.34
Potentially dilutive securities (in shares)	1,223,714	1,040,003	1,176,558	989,495

Net Income (Loss) Available to Common Stockholders, Basic, Net of Participating Securities

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Undistributed Earnings Available to Common Stockholders, Basic Net of Participating Securities

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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Aggregate dividends declared during the period for each share of common stock outstanding.

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The total amount of dividends declared in the period for each class of stock and the contractual amount of dividends (or interest on participating income bonds) that must be paid for the period (for example, unpaid cumulative dividends). Dividends declared in the current period do not include dividends declared in respect of prior-period unpaid cumulative dividends. Preferred dividends that are cumulative only if earned are deducted only to the extent that they are earned.

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Dividends declared in the period and the contractual amount of dividends (or interest on participating income bonds) that must be paid per security.

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The earnings attributable to each security on the basis that all of the earnings for the period had been distributed (to the extent that each security may share in earnings). This is the earnings amount which reflects the participation rights in undistributed earnings of each security.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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