Guarantor and Non-Guarantor Financial Statements - Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	$ (5.9)	$ 1.6
Other comprehensive (loss) income, net of tax:
Unrealized foreign currency translation gain	(16.8)	(18.3)
Amounts reclassified out of AOCI during the period	0.2	0.0
Total other comprehensive (loss) income for the period	(9.2)	14.6
Comprehensive (loss) income	(15.1)	16.2
Comprehensive loss (income) attributable to noncontrolling interests	1.8	(4.6)
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(13.3)	11.6
Reportable Legal Entities [Member] | Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	(6.0)	1.6
Other comprehensive (loss) income, net of tax:
Unrealized foreign currency translation gain	(16.8)	(18.3)
Amounts reclassified out of AOCI during the period	0.2
Total other comprehensive (loss) income for the period	(9.2)	14.6
Comprehensive (loss) income	(15.2)	16.2
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(15.2)	16.2
Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	2.8	12.0
Other comprehensive (loss) income, net of tax:
Unrealized foreign currency translation gain	(16.8)	(18.3)
Amounts reclassified out of AOCI during the period	0.2
Total other comprehensive (loss) income for the period	(9.2)	14.6
Comprehensive (loss) income	(6.4)	26.6
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(6.4)	26.6
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	33.9	50.7
Other comprehensive (loss) income, net of tax:
Unrealized foreign currency translation gain	(49.3)	7.9
Amounts reclassified out of AOCI during the period	0.0
Total other comprehensive (loss) income for the period	(1.0)	(0.1)
Comprehensive (loss) income	32.9	50.6
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	32.9	50.6
Reportable Legal Entities [Member] | Non-guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	45.7	56.0
Other comprehensive (loss) income, net of tax:
Unrealized foreign currency translation gain	(15.7)	(18.3)
Amounts reclassified out of AOCI during the period	0.2
Total other comprehensive (loss) income for the period	(50.8)	22.6
Comprehensive (loss) income	(5.1)	78.6
Comprehensive loss (income) attributable to noncontrolling interests	1.8	(4.6)
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(3.3)	74.0
Elimination [Member]
Condensed Financial Statements, Captions [Line Items]
Net (loss) income	(82.3)	(118.7)
Other comprehensive (loss) income, net of tax:
Unrealized foreign currency translation gain	81.8	28.7
Amounts reclassified out of AOCI during the period	(0.4)
Total other comprehensive (loss) income for the period	61.0	(37.1)
Comprehensive (loss) income	(21.3)	(155.8)
Comprehensive loss (income) attributable to noncontrolling interests	0.0	0.0
Comprehensive income (loss) attributable to Kennedy-Wilson Holdings, Inc.	(21.3)	(155.8)
Foreign Exchange Contract [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	13.0	32.9
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Parent [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	13.0	32.9
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	13.0	32.9
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	48.3	(8.0)
Foreign Exchange Contract [Member] | Reportable Legal Entities [Member] | Non-guarantor Subsidiaries [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	(35.3)	40.9
Foreign Exchange Contract [Member] | Elimination [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	(26.0)	(65.8)
Interest Rate Swap [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	(5.6)	$ 0.0
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Parent [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	(5.6)
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	(5.6)
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	0.0
Interest Rate Swap [Member] | Reportable Legal Entities [Member] | Non-guarantor Subsidiaries [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	0.0
Interest Rate Swap [Member] | Elimination [Member]
Other comprehensive (loss) income, net of tax:
Unrealized gain (loss)	$ 5.6