Variable Interest Entities - Consolidated VIEs (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Carrying amounts of consolidated VIE assets and liabilities
Total assets	$ 276,087.8	$ 243,036.1
Total liabilities	261,137.1	231,188.9
Redeemable noncontrolling interest	264.9	391.2	$ 101.3	$ 97.5
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Total assets	40,802.1	38,922.8
Total liabilities	39,713.7	37,742.9
Redeemable noncontrolling interest	215.4	325.7
Grantor trust
Carrying amounts of consolidated VIE assets and liabilities
Total assets	99.9	95.0
Total liabilities	98.6	89.4
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Total assets		6.4
Mandatory retirement savings funds
Carrying amounts of consolidated VIE assets and liabilities
Total assets	39,891.1	37,915.7
Total liabilities	39,524.9	37,579.3
Real estate VIE
Carrying amounts of consolidated VIE assets and liabilities
Total assets	479.7	379.2
Total liabilities	88.0	70.6
Sponsored investment funds
Carrying amounts of consolidated VIE assets and liabilities
Total assets	331.4	526.5
Total liabilities	2.2	3.6
Redeemable noncontrolling interest	$ 215.4	$ 325.7