Guarantor and Non-Guarantor Financial Statements - Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 167.3	$ 21.8	$ 166.3	$ 22.8
Other comprehensive income (loss), net of tax:
Unrealized gain on marketable securities	0.0	0.1	0.0	0.1
Unrealized foreign currency translation gain	(74.3)	99.6	(38.9)	120.8
Amounts reclassified out of AOCI during the period	13.2	0.0	13.1	0.0
Unrealized currency derivative contracts gain (loss)	29.6	(41.5)	22.2	(32.8)
Total other comprehensive (loss) income for the period	(31.5)	58.2	(3.6)	88.1
Comprehensive income	135.8	80.0	162.7	110.9
Comprehensive income attributable to noncontrolling interests	(58.7)	(61.0)	(65.1)	(85.9)
Comprehensive income attributable to Kennedy-Wilson Holdings, Inc. common shareholders	77.1	19.0	97.6	25.0
Reportable Legal Entities [Member] | Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	167.3	21.8	166.3	22.8
Other comprehensive income (loss), net of tax:
Unrealized gain on marketable securities		0.1		0.1
Unrealized foreign currency translation gain	(74.3)	99.6	(38.9)	120.8
Amounts reclassified out of AOCI during the period	13.2		13.1
Unrealized currency derivative contracts gain (loss)	29.6	(41.5)	22.2	(32.8)
Total other comprehensive (loss) income for the period	(31.5)	58.2	(3.6)	88.1
Comprehensive income	135.8	80.0	162.7	110.9
Comprehensive income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income attributable to Kennedy-Wilson Holdings, Inc. common shareholders	135.8	80.0	162.7	110.9
Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	176.1	31.2	185.0	42.9
Other comprehensive income (loss), net of tax:
Unrealized gain on marketable securities		0.1		0.1
Unrealized foreign currency translation gain	(74.3)	99.6	(38.9)	120.8
Amounts reclassified out of AOCI during the period	13.2		13.1
Unrealized currency derivative contracts gain (loss)	29.6	(41.5)	22.2	(32.8)
Total other comprehensive (loss) income for the period	(31.5)	58.2	(3.6)	88.1
Comprehensive income	144.6	89.4	181.4	131.0
Comprehensive income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income attributable to Kennedy-Wilson Holdings, Inc. common shareholders	144.6	89.4	181.4	131.0
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	230.7	76.1	276.6	107.7
Other comprehensive income (loss), net of tax:
Unrealized gain on marketable securities		0.0		0.0
Unrealized foreign currency translation gain	(34.7)	14.6	(16.3)	18.5
Amounts reclassified out of AOCI during the period	0.0		0.0
Unrealized currency derivative contracts gain (loss)	34.5	(7.8)	16.3	(9.8)
Total other comprehensive (loss) income for the period	(0.2)	6.8	0.0	8.7
Comprehensive income	230.5	82.9	276.6	116.4
Comprehensive income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income attributable to Kennedy-Wilson Holdings, Inc. common shareholders	230.5	82.9	276.6	116.4
Reportable Legal Entities [Member] | Non-guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	176.0	39.7	213.5	66.3
Other comprehensive income (loss), net of tax:
Unrealized gain on marketable securities		0.0		0.0
Unrealized foreign currency translation gain	(72.8)	92.9	(37.4)	112.2
Amounts reclassified out of AOCI during the period	18.3		18.3
Unrealized currency derivative contracts gain (loss)	(4.9)	(33.7)	5.9	(23.0)
Total other comprehensive (loss) income for the period	(59.4)	59.2	(13.2)	89.2
Comprehensive income	116.6	98.9	200.3	155.5
Comprehensive income attributable to noncontrolling interests	(58.7)	(61.0)	(65.1)	(85.9)
Comprehensive income attributable to Kennedy-Wilson Holdings, Inc. common shareholders	57.9	37.9	135.2	69.6
Elimination [Member]
Condensed Financial Statements, Captions [Line Items]
Net income	(582.8)	(147.0)	(675.1)	(216.9)
Other comprehensive income (loss), net of tax:
Unrealized gain on marketable securities		(0.1)		(0.1)
Unrealized foreign currency translation gain	181.8	(207.1)	92.6	(251.5)
Amounts reclassified out of AOCI during the period	(31.5)		(31.4)
Unrealized currency derivative contracts gain (loss)	(59.2)	83.0	(44.4)	65.6
Total other comprehensive (loss) income for the period	91.1	(124.2)	16.8	(186.0)
Comprehensive income	(491.7)	(271.2)	(658.3)	(402.9)
Comprehensive income attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income attributable to Kennedy-Wilson Holdings, Inc. common shareholders	$ (491.7)	$ (271.2)	$ (658.3)	$ (402.9)