Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012		Dec. 31, 2011
Fair Value
Investment securities
Fair value investments	$ 6,565.5	[1],[2]	$ 6,950.7	[1],[2]	$ 6,571.9	[3],[4]	$ 7,500.8	[3],[4]
Residential Mortgage Backed Securities | Fair Value
Investment securities
Fair value investments	133.6		160.6
Residential Mortgage Backed Securities | Prime | Fair Value
Investment securities
Fair value investments	102.7
Residential Mortgage Backed Securities | Non-prime | Fair Value
Investment securities
Fair value investments	30.9
Residential Mortgage Backed Securities | Sub-prime | Fair Value
Investment securities
Fair value investments	0
Residential Mortgage Backed Securities | Super Senior
Investment securities
Fair value investments	27.8	[5]
Residential Mortgage Backed Securities | Super Senior | Prime
Investment securities
Fair value investments	27.8	[5]
Residential Mortgage Backed Securities | Super Senior | Non-prime
Investment securities
Fair value investments	0	[5]
Residential Mortgage Backed Securities | Super Senior | Sub-prime
Investment securities
Fair value investments	0	[5]
Residential Mortgage Backed Securities | Senior
Investment securities
Fair value investments	105.8	[6]
Residential Mortgage Backed Securities | Senior | Prime
Investment securities
Fair value investments	74.9	[6]
Residential Mortgage Backed Securities | Senior | Non-prime
Investment securities
Fair value investments	30.9	[6]
Residential Mortgage Backed Securities | Senior | Sub-prime
Investment securities
Fair value investments	0	[6]
Residential Mortgage Backed Securities | Subordinate
Investment securities
Fair value investments	0	[7]
Residential Mortgage Backed Securities | Subordinate | Prime
Investment securities
Fair value investments	0	[7]
Residential Mortgage Backed Securities | Subordinate | Non-prime
Investment securities
Fair value investments	0	[7]
Residential Mortgage Backed Securities | Subordinate | Sub-prime
Investment securities
Fair value investments	$ 0	[7]

[1] Carrying value includes $260.8 that is classified as assets held for sale relating to discontinued operations.

[2] Excludes carrying value of $34.2 associated with other long-term investment limited partnerships accounted for using the equity method and $(0.1) related to forward contracts.

[3] Carrying value includes $111.8 that is classified as assets held for sale relating to discontinued operations.

[4] Excludes carrying value of $33.0 and $34.0 at January 1, 2012 and June 30, 2012 associated with other long-term investment limited partnerships accounted for using the equity method.

[5] At issuance, Super Senior were rated AAA by Standard & Poor’s, Aaa by Moody’s or AAA by Fitch and were senior to other AAA or Aaa bonds.

[6] At issuance, Senior were rated AAA by Standard & Poor’s, Aaa by Moody’s or AAA by Fitch and were senior to non-AAA or non-Aaa bonds.

[7] At issuance, Subordinate were not rated AAA by Standard & Poor’s, Aaa by Moody’s or AAA by Fitch and were junior to AAA or Aaa bonds.