GUARANTOR AND NON-GUARANTOR FINANCIAL STATEMENTS - Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 73.0	$ 102.9	$ 178.2
Cash flows from investing activities:
Additions to loans		(16.1)	(235.8)
Collections of loans	16.9	146.9	27.8
Nonrefundable escrow deposits	0.0	0.0	(3.8)
Net proceeds from sale of real estate	659.1	540.1	622.6
Purchases of and additions to real estate	(814.3)	(949.6)	(1,927.6)
Investment in marketable securities	(0.8)	(3.2)	(5.0)
Proceeds from sale of marketable securities	1.1	0.0	6.2
Investing distributions from unconsolidated investments	133.4	89.6	180.0
Contributions to unconsolidated investments	(79.9)	(124.9)	(184.3)
Proceeds from settlement of foreign currency derivative contracts	(2.8)	38.6	41.5
Purchases of foreign currency derivative contracts	(0.4)	(8.1)	(5.2)
Additions to development project asset	(19.4)	0.0	0.0
Proceeds from development project asset	36.9	0.0	0.0
(Investments in) distributions from consolidated subsidiaries, net	0.0	0.0	0.0
Net cash used in investing activities	(70.2)	(286.7)	(1,483.6)
Cash flow from financing activities:
Borrowings under senior notes payable	0.0	250.0	0.0
Repayment of senior notes payable	(55.0)	0.0	0.0
Costs associated with KWE Transaction	(55.8)	0.0	0.0
Borrowings under lines of credit	800.0	125.0	95.0
Repayment of lines of credit	(500.0)	(125.0)	(220.0)
Borrowings under investment debt	848.3	1,189.7	2,087.9
Repayment of investment debt	(684.7)	(608.1)	(681.1)
Debt issue costs	(9.5)	(10.8)	(21.7)
Issuance of common stock	0.0	0.0	215.0
Repurchase of common stock	(67.7)	(64.8)	(11.4)
Cash paid to acquire noncontrolling interest in KWE	(719.8)	0.0	0.0
Dividends paid	(59.2)	(64.8)	(51.8)
Acquisition of noncontrolling interests	(3.3)	(196.9)	(68.4)
Contributions from noncontrolling interests	47.4	42.1	14.5
Distributions to noncontrolling interests	(106.0)	(116.6)	(239.2)
Net cash (used in) provided by financing activities	(565.3)	419.8	1,118.8
Effect of currency exchange rate changes on cash and cash equivalents	28.1	(81.9)	(19.5)
Net change in cash and cash equivalents	(534.4)	154.1	(206.1)
Cash and cash equivalents, beginning of year	885.7	731.6	937.7
Cash and cash equivalents, end of year	351.3	885.7	731.6
Reportable Legal Entities [Member] | Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	(15.3)	(0.2)	5.3
Cash flows from investing activities:
Additions to loans		0.0	0.0
Collections of loans	0.0	0.0	0.0
Nonrefundable escrow deposits			0.0
Net proceeds from sale of real estate	0.0	0.0	0.0
Purchases of and additions to real estate	0.0	0.0	0.0
Investment in marketable securities	0.0	0.0	0.0
Proceeds from sale of marketable securities	0.0		0.0
Investing distributions from unconsolidated investments	0.0	0.0	0.0
Contributions to unconsolidated investments	0.0	0.0	0.0
Proceeds from settlement of foreign currency derivative contracts	0.0	0.0	0.0
Purchases of foreign currency derivative contracts	0.0	0.0	0.0
Additions to development project asset	0.0
Proceeds from development project asset	0.0
(Investments in) distributions from consolidated subsidiaries, net	198.0	129.8	(157.1)
Net cash used in investing activities	198.0	129.8	(157.1)
Cash flow from financing activities:
Borrowings under senior notes payable		0.0
Repayment of senior notes payable	0.0
Costs associated with KWE Transaction	(55.8)
Borrowings under lines of credit	0.0	0.0	0.0
Repayment of lines of credit	0.0	0.0	0.0
Borrowings under investment debt	0.0	0.0	0.0
Repayment of investment debt	0.0	0.0	0.0
Debt issue costs	0.0	0.0	0.0
Issuance of common stock			215.0
Repurchase of common stock	(67.7)	(64.8)	(11.4)
Cash paid to acquire noncontrolling interest in KWE	0.0
Dividends paid	(59.2)	(64.8)	(51.8)
Acquisition of noncontrolling interests	0.0	0.0	0.0
Contributions from noncontrolling interests	0.0	0.0	0.0
Distributions to noncontrolling interests	0.0	0.0	0.0
Net cash (used in) provided by financing activities	(182.7)	(129.6)	151.8
Effect of currency exchange rate changes on cash and cash equivalents	0.0	0.0	0.0
Net change in cash and cash equivalents	0.0	0.0	0.0
Cash and cash equivalents, beginning of year	0.0	0.0	0.0
Cash and cash equivalents, end of year	0.0	0.0	0.0
Reportable Legal Entities [Member] | Kennedy-Wilson, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	(182.8)	(207.9)	67.4
Cash flows from investing activities:
Additions to loans		(11.6)	(41.0)
Collections of loans	0.0	6.6	0.0
Nonrefundable escrow deposits			0.0
Net proceeds from sale of real estate	0.0	0.0	0.0
Purchases of and additions to real estate	0.0	0.0	0.0
Investment in marketable securities	0.0	0.0	0.0
Proceeds from sale of marketable securities	0.0		0.0
Investing distributions from unconsolidated investments	0.0		0.0
Contributions to unconsolidated investments	0.0	(1.0)	(2.9)
Proceeds from settlement of foreign currency derivative contracts	(2.8)	38.6	23.4
Purchases of foreign currency derivative contracts	(0.4)	(8.1)	(3.6)
Additions to development project asset	0.0
Proceeds from development project asset	0.0
(Investments in) distributions from consolidated subsidiaries, net	(126.2)	(36.9)	128.4
Net cash used in investing activities	(129.4)	(12.4)	104.3
Cash flow from financing activities:
Borrowings under senior notes payable		250.0
Repayment of senior notes payable	(55.0)
Costs associated with KWE Transaction	0.0
Borrowings under lines of credit	800.0	125.0	95.0
Repayment of lines of credit	(500.0)	(125.0)	(220.0)
Borrowings under investment debt	0.0	0.0	0.0
Repayment of investment debt	0.0	0.0	0.0
Debt issue costs	(5.4)	(3.9)	(4.7)
Issuance of common stock			0.0
Repurchase of common stock	0.0	0.0	0.0
Cash paid to acquire noncontrolling interest in KWE	0.0
Dividends paid	0.0	0.0	0.0
Acquisition of noncontrolling interests	0.0	0.0	0.0
Contributions from noncontrolling interests	0.0	0.0	0.0
Distributions to noncontrolling interests	0.0	0.0	0.0
Net cash (used in) provided by financing activities	239.6	246.1	(129.7)
Effect of currency exchange rate changes on cash and cash equivalents	0.0	0.0	0.0
Net change in cash and cash equivalents	(72.6)	25.8	42.0
Cash and cash equivalents, beginning of year	106.0	80.2	38.2
Cash and cash equivalents, end of year	33.4	106.0	80.2
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	(51.4)	102.2	105.4
Cash flows from investing activities:
Additions to loans		(4.5)	0.0
Collections of loans	6.4	4.9	8.8
Nonrefundable escrow deposits			0.0
Net proceeds from sale of real estate	130.8	109.2	0.0
Purchases of and additions to real estate	(422.5)	(404.2)	(220.3)
Investment in marketable securities	(0.8)	(3.2)	(5.0)
Proceeds from sale of marketable securities	1.1		6.2
Investing distributions from unconsolidated investments	95.2	54.5	83.1
Contributions to unconsolidated investments	(49.8)	(104.4)	(81.5)
Proceeds from settlement of foreign currency derivative contracts	0.0	0.0	0.0
Purchases of foreign currency derivative contracts	0.0	0.0	0.0
Additions to development project asset	0.0
Proceeds from development project asset	0.0
(Investments in) distributions from consolidated subsidiaries, net	166.3	(95.4)	(40.0)
Net cash used in investing activities	(73.3)	(443.1)	(248.7)
Cash flow from financing activities:
Borrowings under senior notes payable		0.0
Repayment of senior notes payable	0.0
Costs associated with KWE Transaction	0.0
Borrowings under lines of credit	0.0	0.0	0.0
Repayment of lines of credit	0.0	0.0	0.0
Borrowings under investment debt	239.5	530.9	168.1
Repayment of investment debt	(103.4)	(179.5)	(7.4)
Debt issue costs	(1.9)	(2.1)	(1.4)
Issuance of common stock			0.0
Repurchase of common stock	0.0	0.0	0.0
Cash paid to acquire noncontrolling interest in KWE	0.0
Dividends paid	0.0	0.0	0.0
Acquisition of noncontrolling interests	0.0	0.0	0.0
Contributions from noncontrolling interests	0.0	0.0	0.0
Distributions to noncontrolling interests	0.0	0.0	0.0
Net cash (used in) provided by financing activities	134.2	349.3	159.3
Effect of currency exchange rate changes on cash and cash equivalents	0.0	0.0	0.0
Net change in cash and cash equivalents	9.5	8.4	16.0
Cash and cash equivalents, beginning of year	45.4	37.0	21.0
Cash and cash equivalents, end of year	54.9	45.4	37.0
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	322.5	208.8	0.1
Cash flows from investing activities:
Additions to loans		0.0	(194.8)
Collections of loans	10.5	135.4	19.0
Nonrefundable escrow deposits			(3.8)
Net proceeds from sale of real estate	528.3	430.9	622.6
Purchases of and additions to real estate	(391.8)	(545.4)	(1,707.3)
Investment in marketable securities	0.0	0.0	0.0
Proceeds from sale of marketable securities	0.0		0.0
Investing distributions from unconsolidated investments	38.2	35.1	96.9
Contributions to unconsolidated investments	(30.1)	(19.5)	(99.9)
Proceeds from settlement of foreign currency derivative contracts	0.0	0.0	18.1
Purchases of foreign currency derivative contracts	0.0	0.0	(1.6)
Additions to development project asset	(19.4)
Proceeds from development project asset	36.9
(Investments in) distributions from consolidated subsidiaries, net	(238.1)	2.5	68.7
Net cash used in investing activities	(65.5)	39.0	(1,182.1)
Cash flow from financing activities:
Borrowings under senior notes payable		0.0
Repayment of senior notes payable	0.0
Costs associated with KWE Transaction	0.0
Borrowings under lines of credit	0.0	0.0	0.0
Repayment of lines of credit	0.0	0.0	0.0
Borrowings under investment debt	608.8	658.8	1,919.8
Repayment of investment debt	(581.3)	(428.6)	(673.7)
Debt issue costs	(2.2)	(4.8)	(15.6)
Issuance of common stock			0.0
Repurchase of common stock	0.0	0.0	0.0
Cash paid to acquire noncontrolling interest in KWE	(719.8)
Dividends paid	0.0	0.0	0.0
Acquisition of noncontrolling interests	(3.3)	(196.9)	(68.4)
Contributions from noncontrolling interests	47.4	42.1	14.5
Distributions to noncontrolling interests	(106.0)	(116.6)	(239.2)
Net cash (used in) provided by financing activities	(756.4)	(46.0)	937.4
Effect of currency exchange rate changes on cash and cash equivalents	28.1	(81.9)	(19.5)
Net change in cash and cash equivalents	(471.3)	119.9	(264.1)
Cash and cash equivalents, beginning of year	734.3	614.4	878.5
Cash and cash equivalents, end of year	$ 263.0	$ 734.3	$ 614.4