Related Parties (Details) - USD ($) $ in Millions	12 Months Ended			19 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 1,408.4	$ 1,049.0	$ 1,147.7	$ 1,408.4
Unsettled fund receivables	109.4	162.7		109.4
Accounts receivable	741.1	855.6		741.1
Investments	826.8	829.5		826.8
Assets held for policyholders	7,582.1	10,835.6		7,582.1
Other assets	514.2	459.6		514.2
Total assets	36,504.1	39,420.3		36,504.1
Accrued compensation and benefits	973.7	1,030.7		973.7
Accounts payable and accrued expenses	1,920.4	1,904.0		1,920.4
Unsettled fund payables	98.4	154.2		98.4
Total liabilities	21,483.4	24,718.5		21,483.4
Affiliated entity
Variable Interest Entity [Line Items]
Total operating revenues	5,606.2	5,536.3	4,747.4
Cash and cash equivalents	947.3	620.9		947.3
Unsettled fund receivables	62.9	113.6		62.9
Accounts receivable	498.3	599.8		498.3
Investments	654.8	633.5		654.8
Assets held for policyholders	7,581.8	10,835.3		7,581.8
Other assets	50.8	24.5		50.8
Total assets	9,795.9	12,827.6		9,795.9
Accrued compensation and benefits	47.4	65.7		47.4
Accounts payable and accrued expenses	60.1	53.8		60.1
Unsettled fund payables	64.1	116.6		64.1
Total liabilities	171.6	236.1		171.6
Investment management fees | Affiliated entity
Variable Interest Entity [Line Items]
Total operating revenues	4,007.8	4,039.7	3,591.7
Service and distribution fees | Affiliated entity
Variable Interest Entity [Line Items]
Total operating revenues	1,358.8	1,219.1	945.8
Performance fees | Affiliated entity
Variable Interest Entity [Line Items]
Total operating revenues	39.4	58.0	17.5
Other | Affiliated entity
Variable Interest Entity [Line Items]
Total operating revenues	200.2	$ 219.5	$ 192.4
OppenheimerFunds Acquisition | Preferred Shares
Variable Interest Entity [Line Items]
Stock and other consideration outstanding	$ 4,000.0			$ 4,010.5
MassMutual | OppenheimerFunds Acquisition | Preferred Shares
Variable Interest Entity [Line Items]
Stake in common stock of combined frim (percent)	16.50%			16.50%