Related Parties (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Total operating revenues	$ 1,341.8	$ 1,337.7	$ 4,058.2	$ 3,784.7
Cash and cash equivalents	1,631.5	1,716.3	1,631.5	1,716.3	$ 2,006.4
Unsettled fund receivables	851.6		851.6		793.8
Accounts receivable	557.5		557.5		622.5
Investments	643.8		643.8		674.6
Other assets	69.0		69.0		61.7
Total assets	32,503.8		32,503.8		31,668.8
Accrued compensation and benefits	552.7		552.7		696.1
Accounts payable and accrued expenses	864.2		864.2		895.7
Unsettled fund payables	821.5		821.5		783.8
Total liabilities	22,761.8		22,761.8		22,470.0
Affiliated Entity [Member]
Total operating revenues	1,202.9	1,209.6	3,636.9	3,386.7
Cash and cash equivalents	404.2		404.2		875.5
Unsettled fund receivables	376.2		376.2		204.0
Accounts receivable	342.5		342.5		359.9
Investments	602.6		602.6		608.5
Separate Account Assets	12,267.8		12,267.8		12,444.2
Other assets	3.4		3.4		9.2
Total assets	13,996.7		13,996.7		14,501.3
Accrued compensation and benefits	74.7		74.7		90.7
Accounts payable and accrued expenses	64.3		64.3		64.5
Unsettled fund payables	184.2		184.2		288.8
Total liabilities	323.2		323.2		$ 444.0
Investment management fees
Total operating revenues	1,038.9	1,062.3	3,133.1	3,027.9
Investment management fees | Affiliated Entity [Member]
Total operating revenues	915.6	937.0	2,757.1	2,654.5
Service and distribution fees
Total operating revenues	248.0	217.6	737.0	635.3
Service and distribution fees | Affiliated Entity [Member]
Total operating revenues	238.9	217.2	723.9	634.1
Performance fees
Total operating revenues	7.9	42.3	28.6	70.3
Performance fees | Affiliated Entity [Member]
Total operating revenues	0.8	40.5	7.6	52.1
Other
Total operating revenues	47.0	15.5	159.5	51.2
Other | Affiliated Entity [Member]
Total operating revenues	$ 47.6	$ 14.9	$ 148.3	$ 46.0