Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details) (Fair Value, USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012		Dec. 31, 2011
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]
Fixed rate CMBS
Investment securities
Fair value investments	199.6
Fixed rate CMBS | Super Senior
Investment securities
Fair value investments	123.5	[5]
Fixed rate CMBS | Senior
Investment securities
Fair value investments	58.7	[6]
Fixed rate CMBS | Subordinate
Investment securities
Fair value investments	17.4	[7]
Floating rate CMBS
Investment securities
Fair value investments	100.7
Floating rate CMBS | Super Senior
Investment securities
Fair value investments	11.6	[5]
Floating rate CMBS | Senior
Investment securities
Fair value investments	36.3	[6]
Floating rate CMBS | Subordinate
Investment securities
Fair value investments	52.8	[7]
Commercial Mortgage Backed Securities
Investment securities
Fair value investments	300.3		334.1
Commercial Mortgage Backed Securities | Super Senior
Investment securities
Fair value investments	135.1	[5]
Commercial Mortgage Backed Securities | Senior
Investment securities
Fair value investments	95.0	[6]
Commercial Mortgage Backed Securities | Subordinate
Investment securities
Fair value investments	$ 70.2	[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.