Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment securities
Fair value investments	$ 5,991.4	$ 6,766.5	$ 6,950.7
Residential
Investment securities
Fair value investments	131.2	125.7
Residential | Prime
Investment securities
Fair value investments	119.4
Residential | Non-prime
Investment securities
Fair value investments	9.3
Residential | Sub-prime
Investment securities
Fair value investments	2.5
Residential | Super Senior
Investment securities
Fair value investments	74.1
Residential | Super Senior | Prime
Investment securities
Fair value investments	71.6
Residential | Super Senior | Non-prime
Investment securities
Fair value investments	0
Residential | Super Senior | Sub-prime
Investment securities
Fair value investments	2.5
Residential | Senior
Investment securities
Fair value investments	57.1
Residential | Senior | Prime
Investment securities
Fair value investments	47.8
Residential | Senior | Non-prime
Investment securities
Fair value investments	9.3
Residential | Senior | Sub-prime
Investment securities
Fair value investments	0
Residential | Subordinate
Investment securities
Fair value investments	0
Residential | Subordinate | Prime
Investment securities
Fair value investments	0
Residential | Subordinate | Non-prime
Investment securities
Fair value investments	0
Residential | Subordinate | Sub-prime
Investment securities
Fair value investments	$ 0