Earnings and Dividends Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings (loss) per share
Net income (loss) attributable to JHG	$ 622.1	$ 161.6	$ 427.6
Allocation of earnings to participating stock-based awards	(17.7)	(4.7)	(11.7)
Net income (loss) attributable to JHG common shareholders	$ 604.4	$ 156.9	$ 415.9
Weighted-average common shares outstanding - basic	167.9	179.4	188.0
Dilutive effect of non-participating stock-based awards	0.6	0.5	0.6
Weighted-average common shares outstanding - diluted	168.5	179.9	188.6
Earnings per share:
Basic (two class)	$ 3.60	$ 0.87	$ 2.21
Diluted (two class)	3.59	0.87	2.21
Dividends Per Share
Dividends paid (per share)	$ 1.50	$ 1.44	$ 1.44
Cash dividends paid	$ 256.0	$ 262.9	$ 272.4