Guarantor and Non-Guarantor Financial Statements - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		$ 59.7	$ 68.2
Cash flows from investing activities:
Additions to loans		(1.5)	0.0
Collections of loans		4.5	6.4
Net proceeds from sale of real estate		834.6	243.0
Purchases of and additions to real estate		(347.8)	(284.9)
(Premiums) proceeds from settlement of foreign derivative contracts		(7.3)	3.0
Additions to nonrefundable escrow deposits			0.0
Purchases of foreign derivative contracts		(0.8)	(0.5)
Investment in marketable securities		0.0	(0.4)
Proceeds from sale of marketable securities		7.4	0.2
Distributions from unconsolidated investments		42.6	69.0
Contributions to unconsolidated investments		(125.6)	(36.7)
Additions to development project asset		(16.4)	0.0
Proceeds from development project asset		38.8	0.0
Distributions from (investments in) consolidated subsidiaries, net		0.0	0.0
Net cash provided by (used in) investing activities		428.5	(0.9)
Cash flows from financing activities:
Borrowings under senior notes payable		246.6	0.0
Borrowings under line of credit		225.0	400.0
Repayment of lines of credit		(325.0)	(50.0)
Borrowings under investment debt		447.2	166.8
Repayment of investment debt		(576.6)	(100.6)
Repayment of term loan		(75.0)	0.0
Debt issue costs		(7.0)	(0.1)
Costs associated with KWE transaction		0.0	(5.5)
Repurchase and retirement of common stock		(155.6)	(35.6)
Dividends paid		(57.5)	(37.9)
KWE closing dividend		(17.2)	0.0
Acquisition of KWE shares from noncontrolling interest holders		0.0	(3.3)
Contributions from noncontrolling interests, excluding KWE		15.2	22.7
Distributions to noncontrolling interests		(103.9)	(76.2)
Net cash (used in) provided by financing activities		(383.8)	280.3
Effect of currency exchange rate changes on cash and cash equivalents		(8.6)	31.5
Net change in cash and cash equivalents	[1]	95.8	379.1
Cash and cash equivalents, beginning of period		351.3	885.7
Cash and cash equivalents, end of period		447.1	1,264.8
Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		0.0	2.3
Cash flows from investing activities:
Additions to loans		0.0	0.0
Collections of loans		0.0	0.0
Net proceeds from sale of real estate		0.0	0.0
Purchases of and additions to real estate		0.0	0.0
(Premiums) proceeds from settlement of foreign derivative contracts		0.0	0.0
Additions to nonrefundable escrow deposits			0.0
Purchases of foreign derivative contracts		0.0	0.0
Investment in marketable securities		0.0	0.0
Proceeds from sale of marketable securities		0.0	0.0
Distributions from unconsolidated investments		0.0	0.0
Contributions to unconsolidated investments		0.0	0.0
Additions to development project asset		0.0
Proceeds from development project asset		0.0
Distributions from (investments in) consolidated subsidiaries, net		213.1	76.7
Net cash provided by (used in) investing activities		213.1	76.7
Cash flows from financing activities:
Borrowings under senior notes payable		0.0
Borrowings under line of credit		0.0	0.0
Repayment of lines of credit		0.0	0.0
Borrowings under investment debt		0.0	0.0
Repayment of investment debt		0.0	0.0
Repayment of term loan		0.0
Debt issue costs		0.0	0.0
Costs associated with KWE transaction			(5.5)
Repurchase and retirement of common stock		(155.6)	(35.6)
Dividends paid		(57.5)	(37.9)
KWE closing dividend		0.0
Acquisition of KWE shares from noncontrolling interest holders			0.0
Contributions from noncontrolling interests, excluding KWE		0.0	0.0
Distributions to noncontrolling interests		0.0	0.0
Net cash (used in) provided by financing activities		(213.1)	(79.0)
Effect of currency exchange rate changes on cash and cash equivalents		0.0	0.0
Net change in cash and cash equivalents		0.0	0.0
Cash and cash equivalents, beginning of period		0.0	0.0
Cash and cash equivalents, end of period		0.0	0.0
Kennedy-Wilson, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		(64.4)	(223.7)
Cash flows from investing activities:
Additions to loans		0.0	0.0
Collections of loans		0.0	0.0
Net proceeds from sale of real estate		0.0	0.0
Purchases of and additions to real estate		0.0	0.0
(Premiums) proceeds from settlement of foreign derivative contracts		(7.3)	3.0
Additions to nonrefundable escrow deposits			0.0
Purchases of foreign derivative contracts		(0.8)	(0.5)
Investment in marketable securities		0.0	0.0
Proceeds from sale of marketable securities		0.0	0.0
Distributions from unconsolidated investments		0.0	0.0
Contributions to unconsolidated investments		(0.6)	0.0
Additions to development project asset		0.0
Proceeds from development project asset		0.0
Distributions from (investments in) consolidated subsidiaries, net		(9.3)	175.7
Net cash provided by (used in) investing activities		(18.0)	178.2
Cash flows from financing activities:
Borrowings under senior notes payable		246.6
Borrowings under line of credit		225.0	400.0
Repayment of lines of credit		(325.0)	(50.0)
Borrowings under investment debt		0.0	0.0
Repayment of investment debt		0.0	0.0
Repayment of term loan		(75.0)
Debt issue costs		(4.0)	(0.1)
Costs associated with KWE transaction			0.0
Repurchase and retirement of common stock		0.0	0.0
Dividends paid		0.0	0.0
KWE closing dividend		0.0
Acquisition of KWE shares from noncontrolling interest holders			0.0
Contributions from noncontrolling interests, excluding KWE		0.0	0.0
Distributions to noncontrolling interests		0.0	0.0
Net cash (used in) provided by financing activities		67.6	349.9
Effect of currency exchange rate changes on cash and cash equivalents		0.0	0.0
Net change in cash and cash equivalents		(14.8)	304.4
Cash and cash equivalents, beginning of period		33.4	106.0
Cash and cash equivalents, end of period		18.6	410.4
Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		29.0	19.1
Cash flows from investing activities:
Additions to loans		(0.6)	0.0
Collections of loans		4.5	6.4
Net proceeds from sale of real estate		161.3	108.4
Purchases of and additions to real estate		(139.4)	(166.8)
(Premiums) proceeds from settlement of foreign derivative contracts		0.0	0.0
Additions to nonrefundable escrow deposits			0.0
Purchases of foreign derivative contracts		0.0	0.0
Investment in marketable securities		0.0	(0.4)
Proceeds from sale of marketable securities		7.4	0.2
Distributions from unconsolidated investments		16.3	53.6
Contributions to unconsolidated investments		(20.3)	(30.2)
Additions to development project asset		0.0
Proceeds from development project asset		0.0
Distributions from (investments in) consolidated subsidiaries, net		(49.1)	(32.8)
Net cash provided by (used in) investing activities		(19.9)	(61.6)
Cash flows from financing activities:
Borrowings under senior notes payable		0.0
Borrowings under line of credit		0.0	0.0
Repayment of lines of credit		0.0	0.0
Borrowings under investment debt		291.7	77.0
Repayment of investment debt		(276.1)	(34.4)
Repayment of term loan		0.0
Debt issue costs		(1.4)	0.0
Costs associated with KWE transaction			0.0
Repurchase and retirement of common stock		0.0	0.0
Dividends paid		0.0	0.0
KWE closing dividend		0.0
Acquisition of KWE shares from noncontrolling interest holders			0.0
Contributions from noncontrolling interests, excluding KWE		0.0	0.0
Distributions to noncontrolling interests		0.0	0.0
Net cash (used in) provided by financing activities		14.2	42.6
Effect of currency exchange rate changes on cash and cash equivalents		0.0	0.0
Net change in cash and cash equivalents		23.3	0.1
Cash and cash equivalents, beginning of period		54.8	45.4
Cash and cash equivalents, end of period		78.1	45.5
Non-guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash (used in) provided by operating activities		95.1	270.5
Cash flows from investing activities:
Additions to loans		(0.9)	0.0
Collections of loans		0.0	0.0
Net proceeds from sale of real estate		673.3	134.6
Purchases of and additions to real estate		(208.4)	(118.1)
(Premiums) proceeds from settlement of foreign derivative contracts		0.0	0.0
Additions to nonrefundable escrow deposits			0.0
Purchases of foreign derivative contracts		0.0	0.0
Investment in marketable securities		0.0	0.0
Proceeds from sale of marketable securities		0.0	0.0
Distributions from unconsolidated investments		26.3	15.4
Contributions to unconsolidated investments		(104.7)	(6.5)
Additions to development project asset		(16.4)
Proceeds from development project asset		38.8
Distributions from (investments in) consolidated subsidiaries, net		(154.7)	(219.6)
Net cash provided by (used in) investing activities		253.3	(194.2)
Cash flows from financing activities:
Borrowings under senior notes payable		0.0
Borrowings under line of credit		0.0	0.0
Repayment of lines of credit		0.0	0.0
Borrowings under investment debt		155.5	89.8
Repayment of investment debt		(300.5)	(66.2)
Repayment of term loan		0.0
Debt issue costs		(1.6)	0.0
Costs associated with KWE transaction			0.0
Repurchase and retirement of common stock		0.0	0.0
Dividends paid		0.0	0.0
KWE closing dividend		(17.2)
Acquisition of KWE shares from noncontrolling interest holders			(3.3)
Contributions from noncontrolling interests, excluding KWE		15.2	22.7
Distributions to noncontrolling interests		(103.9)	(76.2)
Net cash (used in) provided by financing activities		(252.5)	(33.2)
Effect of currency exchange rate changes on cash and cash equivalents		(8.6)	31.5
Net change in cash and cash equivalents		87.3	74.6
Cash and cash equivalents, beginning of period		263.1	734.3
Cash and cash equivalents, end of period		$ 350.4	$ 808.9

[1] See discussion of non-cash effects in notes to consolidated statements of cash flows.