Retirement benefit plans - Plan assets (Details) - Defined benefit pension Plan $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)
Fair value of plan assets
Total assets at fair value	$ 945.9	$ 941.8	$ 945.9
Assets held in JHG managed funds			250.9
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	849.5	941.8
Losses recognized in the period	100.1	(11.1)
Fair value of plan assets at the end of the year	$ 945.9	$ 849.5
Expected rate of return on plan assets (as a percent)	2.5	2.5
Cash and cash equivalents
Fair value of plan assets
Total assets at fair value	$ 6.4	$ 6.4	3.7
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	6.4
Fair value of plan assets at the end of the year	3.7	6.4
Money market instruments
Fair value of plan assets
Total assets at fair value	21.6	21.6	78.1
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	21.6
Fair value of plan assets at the end of the year	78.1	21.6
Forward foreign exchange contracts
Fair value of plan assets
Total assets at fair value	0.3	0.3
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	0.3
Fair value of plan assets at the end of the year		0.3
Bulk annuity policy
Fair value of plan assets
Total assets at fair value	395.8		$ 395.8
Strategic allocation of plan assets			40.00%
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the end of the year	395.8
Fixed income investments
Fair value of plan assets
Total assets at fair value	623.2	623.2	$ 261.4
Strategic allocation of plan assets			30.00%
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	623.2
Fair value of plan assets at the end of the year	261.4	623.2
Equity investments
Fair value of plan assets
Total assets at fair value	198.0	198.0	$ 206.9
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	198.0
Fair value of plan assets at the end of the year	206.9	198.0
Growth portfolio
Fair value of plan assets
Strategic allocation of plan assets			30.00%
Fair value measurements, recurring
Fair value of plan assets
Total assets at fair value	849.5	849.5	$ 945.9
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	849.5
Fair value of plan assets at the end of the year	945.9	849.5
Fair value measurements, recurring | Cash and cash equivalents
Fair value of plan assets
Total assets at fair value	6.4	6.4	3.7
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	6.4
Fair value of plan assets at the end of the year	3.7	6.4
Fair value measurements, recurring | Money market instruments
Fair value of plan assets
Total assets at fair value	21.6	21.6	78.1
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	21.6
Fair value of plan assets at the end of the year	78.1	21.6
Fair value measurements, recurring | Forward foreign exchange contracts
Fair value of plan assets
Total assets at fair value	0.3	0.3
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	0.3
Fair value of plan assets at the end of the year		0.3
Fair value measurements, recurring | Bulk annuity policy
Fair value of plan assets
Total assets at fair value	395.8		395.8
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the end of the year	395.8
Fair value measurements, recurring | Fixed income investments
Fair value of plan assets
Total assets at fair value	623.2	623.2	261.4
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	623.2
Fair value of plan assets at the end of the year	261.4	623.2
Fair value measurements, recurring | Equity investments
Fair value of plan assets
Total assets at fair value	198.0	198.0	206.9
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	198.0
Fair value of plan assets at the end of the year	206.9	198.0
Fair value measurements, recurring | Level 1
Fair value of plan assets
Total assets at fair value	625.7	625.7	472.0
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	625.7
Fair value of plan assets at the end of the year	472.0	625.7
Fair value measurements, recurring | Level 1 | Cash and cash equivalents
Fair value of plan assets
Total assets at fair value	6.4	6.4	3.7
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	6.4
Fair value of plan assets at the end of the year	3.7	6.4
Fair value measurements, recurring | Level 1 | Forward foreign exchange contracts
Fair value of plan assets
Total assets at fair value	0.3	0.3
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	0.3
Fair value of plan assets at the end of the year		0.3
Fair value measurements, recurring | Level 1 | Fixed income investments
Fair value of plan assets
Total assets at fair value	619.0	619.0	261.4
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	619.0
Fair value of plan assets at the end of the year	261.4	619.0
Fair value measurements, recurring | Level 1 | Equity investments
Fair value of plan assets
Total assets at fair value	206.9		206.9
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the end of the year	206.9
Fair value measurements, recurring | Level 2
Fair value of plan assets
Total assets at fair value	223.8	223.8	78.1
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	223.8
Fair value of plan assets at the end of the year	78.1	223.8
Fair value measurements, recurring | Level 2 | Money market instruments
Fair value of plan assets
Total assets at fair value	21.6	21.6	78.1
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	21.6
Fair value of plan assets at the end of the year	78.1	21.6
Fair value measurements, recurring | Level 2 | Fixed income investments
Fair value of plan assets
Total assets at fair value	4.2	4.2
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	4.2
Fair value of plan assets at the end of the year		4.2
Fair value measurements, recurring | Level 2 | Equity investments
Fair value of plan assets
Total assets at fair value	198.0	198.0
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the beginning of the year	198.0
Fair value of plan assets at the end of the year		$ 198.0
Fair value measurements, recurring | Level 3
Fair value of plan assets
Total assets at fair value	395.8		395.8
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the end of the year	395.8
Fair value measurements, recurring | Level 3 | Bulk annuity policy
Fair value of plan assets
Total assets at fair value	395.8		$ 395.8
Change in fair value of plan assets level 3 reconciliation
Fair value of plan assets at the end of the year	$ 395.8