Note 16 - Retirement Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2023

Notes Tables

Defined Benefit Plan, Plan with Projected Benefit Obligation in Excess of Plan Assets [Table Text Block]

	2023			2022
Change in plan assets:
Fair value of plan assets as of January 1	$	660.5		$	1,142.6
Return on plan assets		15.2			(335.0	)
Employer contributions		2.2			2.1
Benefits paid		(17.9	)		(15.8	)
Settlements		(3.2	)		(9.7	)
Foreign currency translation		39.6			(123.7	)
Fair value of plan assets as of December 31		696.4			660.5
Change in benefit obligation:
Benefit obligation as of January 1		(565.6	)		(975.2	)
Interest cost		(27.6	)		(16.9	)
Settlements		3.2			9.7
Benefits paid		17.9			15.8
Actuarial (loss)/gain		(5.1	)		295.4
Foreign currency translation		(33.9	)		105.6
Benefit obligation as of December 31		(611.1	)		(565.6	)
Funded status as of year-end		85.3			94.9
Net retirement benefit asset recognized in the Consolidated Balance Sheets	$	85.3		$	94.9

Schedule of Amounts Recognized in Balance Sheet [Table Text Block]

	December 31,
	2023			2022
Retirement benefit assets recognized in the Consolidated Balance Sheets:
Janus Henderson Group UK Pension Scheme	$	87.6		$	97.9
Retirement benefit obligations recognized in the Consolidated Balance Sheets:		​			​
Janus Henderson Group unapproved pension scheme		(2.3	)		(3.0	)
Net retirement benefit asset recognized in the Consolidated Balance Sheets	$	85.3		$	94.9

Schedule of Assumptions Used in Determining Defined Benefit Obligation [Table Text Block]

	December 31,
	2023			2022
Discount rate		4.5	%		4.8	%
Inflation — Retail Price Index (“RPI”)		3.1	%		3.3	%
Inflation — Consumer Price Index (“CPI”)		2.5	%		2.7	%
Pension increases (RPI capped at 5% per annum (“p.a.”))		3.0	%		3.2	%
Pension increases (RPI capped at 2.5% p.a.)		2.1	%		2.1	%
Life expectancy of male aged 60 at accounting date		29.1			29.4
Life expectancy of male aged 60 in 15 years’ time		30.1			30.3

Schedule of Allocation of Plan Assets [Table Text Block]

	December 31,
	2023		2022
Cash and cash equivalents	$	19.9	$	1.8
Money market instruments		7.8		8.0
Bulk annuity policy		240.7		230.7
Fixed income investments		300.6		249.3
Equity investments		127.4		170.7
Total assets at fair value	$	696.4	$	660.5

As of December 31, 2023 and 2022, $103.4 million and $148.4 million, respectively, of JHGPS assets were held in JHG-managed funds.

Schedule of Defined Benefit Plans Disclosures [Table Text Block]

	Fair value measurements using:
	Quoted prices in
	active markets for
	identical assets		Significant other		Significant
	and liabilities		observable inputs		unobservable inputs
	(Level 1)		(Level 2)		(Level 3)		Total
Cash and cash equivalents	$	19.9	$	—	$	—	$	19.9
Money market instruments		7.8		—		—		7.8
Bulk annuity contract		—		—		240.7		240.7
Fixed income investments		300.6		—		—		300.6
Equity investments		127.4		—		—		127.4
Total	$	455.7	$	—	$	240.7	$	696.4

	Fair value measurements using:
	Quoted prices in
	active markets for
	identical assets		Significant other		Significant
	and liabilities		observable inputs		unobservable inputs
	(Level 1)		(Level 2)		(Level 3)		Total
Cash and cash equivalents	$	1.8	$	—	$	—	$	1.8
Money market instruments		8.0		—		—		8.0
Bulk annuity contract		—		—		230.7		230.7
Fixed income investments		249.3		—		—		249.3
Equity investments		170.7		—		—		170.7
Total	$	429.8	$	—	$	230.7	$	660.5

Schedule of Actuarial Gains and Losses [Table Text Block]

	2023			2022
Opening accumulated unamortized actuarial loss as of January 1	$	(70.4	)	$	(32.8	)
Actuarial loss		(18.8	)		(53.1	)
Tax at source on current year actuarial gain		3.5			15.1
Prior service cost		0.5			0.4
Amortization of actuarial loss		0.5			—
Closing accumulated unamortized actuarial loss as of December 31	$	(84.7	)	$	(70.4	)

Schedule of Assumptions Used in Determining Net Periodic Benefit Cost [Table Text Block]

	December 31,
	2023			2022			2021
Service cost	$	—		$	—		$	(0.6	)
Settlement loss		—			—			(1.1	)
Curtailment loss		—			—			(0.3	)
Interest cost		(27.6	)		(16.9	)		(13.5	)
Amortization of actuarial loss		(0.5	)		—			—
Amortization of prior service cost		(0.4	)		(0.4	)		(0.4	)
Expected return on plan assets		28.7			13.8			11.3
Net periodic benefit credit (cost)		0.2			(3.5	)		(4.6	)
Contributions to money purchase section		(11.6	)		(11.5	)		(11.3	)
Total cost	$	(11.4	)	$	(15.0	)	$	(15.9	)

	December 31,
	2023			2022			2021
Discount rate		4.8	%		1.9	%		1.3	%
Inflation — salaries		N/A			N/A			2.5	%
Inflation — RPI		3.3	%		3.4	%		2.9	%
Inflation — CPI		2.7	%		2.8	%		2.2	%
Pension increases (RPI capped at 5% p.a.)		3.2	%		3.3	%		2.9	%
Pension increases (RPI capped at 2.5% p.a.)		2.1	%		2.2	%		2.1	%
Expected return on plan assets		4.4	%		1.6	%		1.2	%
Amortization period for net actuarial gains at beginning of the year		28.0			29.0			9.0

Cash Flows

Schedule of Expected Benefit Payments [Table Text Block]

2024	$	22.1
2025	$	22.4
2026	$	23.8
2027	$	24.7
2028	$	25.1
2029-2033	$	137.6