Variable Interest Entities (Details 2) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Sponsored Investment Funds
Total assets of unconsolidated money market mutual fund variable interest entities	$ 1,400,000,000	$ 1,500,000,000
Other investments: Other limited partnership interests
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	128,400,000	136,200,000
Maximum exposure to loss	128,400,000	136,200,000
Available-for-sale | Corporate debt securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	527,900,000	523,200,000
Maximum exposure to loss	408,900,000	403,700,000
Available-for-sale | Residential mortgage-backed pass-through securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,123,200,000	3,226,700,000
Maximum exposure to loss	2,950,300,000	3,022,700,000
Available-for-sale | Commercial mortgage-backed securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,990,700,000	3,897,400,000
Maximum exposure to loss	4,110,900,000	4,094,800,000
Available-for-sale | Collateralized debt obligations
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	394,000,000	379,200,000
Maximum exposure to loss	431,700,000	428,800,000
Available-for-sale | Other debt obligations
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,848,600,000	3,779,200,000
Maximum exposure to loss	3,808,400,000	3,756,900,000
Trading | Residential mortgage-backed pass-through securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	71,500,000	77,700,000
Maximum exposure to loss	71,500,000	77,700,000
Trading | Commercial mortgage-backed securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,100,000	2,800,000
Maximum exposure to loss	3,100,000	2,800,000
Trading | Collateralized debt obligations
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	59,200,000	56,400,000
Maximum exposure to loss	59,200,000	56,400,000
Trading | Other debt obligations
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	1,900,000	3,200,000
Maximum exposure to loss	$ 1,900,000	$ 3,200,000