Related Parties (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Related Party Transactions [Line Items]
Total operating revenues	$ 1,659.7	$ 1,598.9
Cash and cash equivalents	1,158.3	940.5	$ 1,408.4
Unsettled fund receivables	347.9		109.4
Accounts receivable	770.8		741.1
Investments	857.6		826.8
Other assets	614.0		514.2
Total assets	34,881.5		36,504.1
Accrued compensation and benefits	592.2		973.7
Accounts payable and accrued expenses	1,930.8		1,920.4
Unsettled fund payables	347.2		98.4
Total liabilities	19,513.7		21,483.4
Affiliated Entity
Related Party Transactions [Line Items]
Total operating revenues	1,519.8	1,465.5
Cash and cash equivalents	609.2		947.3
Unsettled fund receivables	283.3		62.9
Accounts receivable	555.3		498.3
Investments	676.2		654.8
Assets held for policyholders	5,629.2		7,581.8
Other assets	56.4		50.8
Total assets	7,809.6		9,795.9
Accrued compensation and benefits	51.9		47.4
Accounts payable and accrued expenses	65.7		60.1
Unsettled fund payables	282.5		64.1
Total liabilities	400.1		$ 171.6
Investment management fees
Related Party Transactions [Line Items]
Total operating revenues	1,206.6	1,168.3
Investment management fees | Affiliated Entity
Related Party Transactions [Line Items]
Total operating revenues	1,090.0	1,055.9
Service and distribution fees
Related Party Transactions [Line Items]
Total operating revenues	381.1	365.8
Service and distribution fees | Affiliated Entity
Related Party Transactions [Line Items]
Total operating revenues	366.3	349.2
Performance fees
Related Party Transactions [Line Items]
Total operating revenues	6.7	4.8
Performance fees | Affiliated Entity
Related Party Transactions [Line Items]
Total operating revenues	1.6	1.0
Other
Related Party Transactions [Line Items]
Total operating revenues	65.3	60.0
Other | Affiliated Entity
Related Party Transactions [Line Items]
Total operating revenues	$ 61.9	$ 59.4