Variable Interest Entities - Consolidated VIEs (Details) $ in Millions	Mar. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Carrying amounts of consolidated VIE assets and liabilities
Total assets	$ 37,051.4	$ 34,702.2
Total liabilities	$ 36,064.5	34,028.3
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	$ 232.3	257.9
Total liabilities	$ 214.9	231.8
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	$ 100.4	100.4
Total liabilities	84.5	85.9
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Total assets	16.7	18.4
Mandatory retirement savings
Carrying amounts of consolidated VIE assets and liabilities
Total assets	36,023.4	33,941.3
Total liabilities	35,696.0	33,639.3
Real estate VIE
Carrying amounts of consolidated VIE assets and liabilities
Total assets	406.9	384.2
Total liabilities	66.9	$ 71.3
Sponsored investment funds
Carrying amounts of consolidated VIE assets and liabilities
Total assets	271.7
Total liabilities	2.2
Redeemable noncontrolling interest	$ 195.6