Guarantor and Non-Guarantor Financial Statements - Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	$ (19.7)	$ 19.4	$ (64.7)	$ 162.2
Other comprehensive income (loss), net of tax:
Unrealized foreign currency translation gain	28.8	(31.7)	33.5	(36.4)
Amounts reclassified out of AOCI during the period	0.0	0.0	0.2	9.8
Total other comprehensive income (loss) for the period	10.6	(8.3)	2.9	13.9
Comprehensive (loss) income	(9.1)	11.1	(61.8)	176.1
Comprehensive loss (income) attributable to noncontrolling interests	(1.9)	2.3	1.1	(105.0)
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(11.0)	13.4	(60.7)	71.1
Reportable Legal Entities [Member] | Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	(19.7)	19.4	(64.7)	162.2
Other comprehensive income (loss), net of tax:
Unrealized foreign currency translation gain	28.8	(31.7)	33.5	(36.4)
Amounts reclassified out of AOCI during the period	0.0	0.0	0.2	9.8
Total other comprehensive income (loss) for the period	10.5	(8.3)	2.9	13.9
Comprehensive (loss) income	(9.2)	11.1	(61.8)	176.1
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(9.2)	11.1	(61.8)	176.1
Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	(11.8)	26.9	(39.3)	187.3
Other comprehensive income (loss), net of tax:
Unrealized foreign currency translation gain	28.8	(31.8)	33.5	(36.4)
Amounts reclassified out of AOCI during the period	0.0	0.0	0.2	9.8
Total other comprehensive income (loss) for the period	10.5	(8.4)	2.9	13.9
Comprehensive (loss) income	(1.3)	18.5	(36.4)	201.2
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(1.3)	18.5	(36.4)	201.2
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	4.5	76.2	31.9	336.1
Other comprehensive income (loss), net of tax:
Unrealized foreign currency translation gain	19.9	(17.1)	(30.4)	(28.8)
Amounts reclassified out of AOCI during the period	0.0	0.0	0.0	0.0
Total other comprehensive income (loss) for the period	0.8	(0.3)	0.0	0.0
Comprehensive (loss) income	5.3	75.9	31.9	336.1
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	5.3	75.9	31.9	336.1
Reportable Legal Entities [Member] | Non-guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	8.8	(0.7)	59.2	274.6
Other comprehensive income (loss), net of tax:
Unrealized foreign currency translation gain	28.0	(31.5)	33.5	(27.1)
Amounts reclassified out of AOCI during the period	0.0	0.0	0.2	9.8
Total other comprehensive income (loss) for the period	28.4	(23.4)	(21.6)	(3.7)
Comprehensive (loss) income	37.2	(24.1)	37.6	270.9
Comprehensive loss (income) attributable to noncontrolling interests	(1.9)	2.3	1.1	(105.0)
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	35.3	(21.8)	38.7	165.9
Elimination [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	(1.5)	(102.4)	(51.8)	(798.0)
Other comprehensive income (loss), net of tax:
Unrealized foreign currency translation gain	(76.7)	80.4	(36.6)	92.3
Amounts reclassified out of AOCI during the period	0.0	0.0	(0.4)	(19.6)
Total other comprehensive income (loss) for the period	(39.6)	32.1	18.7	(10.2)
Comprehensive (loss) income	(41.1)	(70.3)	(33.1)	(808.2)
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(41.1)	(70.3)	(33.1)	(808.2)
Foreign Exchange Contract [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	(18.6)	24.9	(24.9)	42.4
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Parent [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	(18.7)	24.9	(24.9)	42.4
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	(18.7)	24.9	(24.9)	42.4
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	(19.1)	16.8	30.4	28.8
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Non-guarantor Subsidiaries [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	0.4	8.1	(55.3)	13.6
Foreign Exchange Contract [Member] | Elimination [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	37.5	(49.8)	49.8	(84.8)
Interest Rate Swap [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	0.4	(1.5)	(5.9)	(1.9)
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Parent [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	0.4	(1.5)	(5.9)	(1.9)
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	0.4	(1.5)	(5.9)	(1.9)
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	0.0	0.0	0.0	0.0
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Non-guarantor Subsidiaries [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	0.0	0.0	0.0	0.0
Interest Rate Swap [Member] | Elimination [Member]
Other comprehensive income (loss), net of tax:
Unrealized gain (loss)	$ (0.4)	$ 1.5	$ 5.9	$ 1.9