Related Parties (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investment Advisory Fees	$ 1,043.7	$ 955.2
Distribution and Servicing Fees	246.1	206.4
Performance Fees	9.1	11.3
Revenue, Other Financial Services	56.9	19.7
Revenues	1,355.8	1,192.6
Cash and cash equivalents	1,861.5	1,397.0	$ 2,006.4
Unsettled fund receivables	837.6		793.8
Accounts receivable	595.0		622.5
Investments	713.6		674.6
Separate Account Assets	12,902.2		12,444.5
Other assets	64.5		61.7
Total assets	31,619.1		31,668.8
Accrued compensation and benefits	371.6		696.1
Accounts payable and accrued expenses	820.8		895.7
Unsettled fund payables	811.5		783.8
Total liabilities	22,148.1		22,470.0
Affiliated Entity [Member]
Investment Advisory Fees	916.1	832.1
Distribution and Servicing Fees	245.3	206.1
Performance Fees	4.1	8.3
Revenue, Other Financial Services	54.8	16.8
Revenues	1,220.3	$ 1,063.3
Cash and cash equivalents	818.1		875.5
Unsettled fund receivables	387.1		204.0
Accounts receivable	349.8		359.9
Investments	646.0		608.5
Separate Account Assets	12,901.9		12,444.2
Other assets	3.6		9.2
Total assets	15,106.5		14,501.3
Accrued compensation and benefits	79.2		90.7
Accounts payable and accrued expenses	65.3		64.5
Unsettled fund payables	380.9		288.8
Total liabilities	$ 525.4		$ 444.0