CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue:
Total revenue	$ 518.0	$ 535.9	$ 1,072.9	$ 1,055.2
Operating expenses:
Employee compensation and benefits	145.8	146.5	301.4	291.5
Long-term incentive plans	49.1	49.2	82.7	97.6
Distribution expenses	104.7	101.5	216.9	203.4
Investment administration	12.6	11.1	24.3	22.9
Marketing	3.7	8.1	10.4	15.6
General, administrative and occupancy	58.0	67.7	123.2	132.9
Impairment of goodwill and intangible assets	26.4	18.0	513.7	18.0
Depreciation and amortization	11.0	15.3	26.0	30.3
Total operating expenses	411.3	417.4	1,298.6	812.2
Operating income (loss)	106.7	118.5	(225.7)	243.0
Interest expense	(3.2)	(4.2)	(6.5)	(8.3)
Investment gains (losses), net	50.3	4.8	(0.2)	18.1
Other non-operating income, net	8.6	28.5	40.8	24.6
Income (loss) before taxes	162.4	147.6	(191.6)	277.4
Income tax benefit (provision)	(30.1)	(35.3)	38.7	(65.2)
Net income (loss)	132.3	112.3	(152.9)	212.2
Net loss (income) attributable to noncontrolling interests	(29.4)	(2.9)	8.8	(8.7)
Net income (loss) attributable to JHG	$ 102.9	$ 109.4	$ (144.1)	$ 203.5
Earnings (loss) per share attributable to JHG common shareholders:
Basic (in dollars per share)	$ 0.55	$ 0.56	$ (0.79)	$ 1.04
Diluted (in dollars per share)	$ 0.55	$ 0.56	$ (0.79)	$ 1.03
Other comprehensive income (loss), net of tax:
Foreign currency translation gains (losses)	$ 10.6	$ (37.3)	$ (138.5)	$ (0.5)
Actuarial gains	0.1		0.2
Total other comprehensive income (loss)	10.7	(37.3)	(138.3)	(0.5)
Other comprehensive loss (income) attributable to noncontrolling interests	(1.4)	0.1	3.7
Other comprehensive income (loss) attributable to JHG	9.3	(37.2)	(134.6)	(0.5)
Total comprehensive income (loss)	143.0	75.0	(291.2)	211.7
Total comprehensive loss (income) attributable to noncontrolling interests	(30.8)	(2.8)	12.5	(8.7)
Total comprehensive income (loss) attributable to JHG	112.2	72.2	(278.7)	203.0
Management fees
Revenue:
Total revenue	407.7	446.4	847.3	888.3
Performance fees
Revenue:
Total revenue	17.2	3.5	31.8	(2.1)
Shareowner servicing fees
Revenue:
Total revenue	47.3	38.3	97.6	74.2
Other revenue
Revenue:
Total revenue	$ 45.8	$ 47.7	$ 96.2	$ 94.8