Variable Interest Entities - Consolidated VIEs (Details) $ in Millions	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Carrying amounts of consolidated VIE assets and liabilities
Total assets	$ 43,154.8	$ 37,298.4
Total liabilities	$ 42,195.0	36,504.3
Grantor trusts
Consolidated Variable Interest Entity disclosures
Number of consolidated variable interest entities | item	3
Carrying amounts of consolidated VIE assets and liabilities
Total assets	$ 268.8	233.3
Total liabilities	253.1	$ 212.3
Collateralized private investment vehicle
Consolidated Variable Interest Entity disclosures
Number of entities for which the reporting entity acts as primary beneficiary | item		1
Carrying amounts of consolidated VIE assets and liabilities
Total assets		$ 82.4
Total liabilities		61.5
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Total assets	9.4	12.5
Mandatory retirement savings funds
Carrying amounts of consolidated VIE assets and liabilities
Total assets	42,311.4	36,526.7
Total liabilities	41,921.4	36,202.8
Real estate VIE
Carrying amounts of consolidated VIE assets and liabilities
Total assets	387.1	329.2
Total liabilities	19.5	26.8
Sponsored investment funds
Carrying amounts of consolidated VIE assets and liabilities
Total assets	178.1	114.3
Total liabilities	1.0	0.9
Redeemable noncontrolling interest	$ 52.4	$ 58.8