Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment securities
Fair value investments	$ 4,050.9	$ 3,343.6	$ 3,889.0
Residential
Investment securities
Fair value investments	133.2	$ 71.0
Residential | Prime [Member]
Investment securities
Fair value investments	133.1
Residential | Non-prime
Investment securities
Fair value investments	0.1
Residential | Subprime [Member]
Investment securities
Fair value investments	0.0
Residential | Super Senior
Investment securities
Fair value investments	65.6
Residential | Super Senior | Prime [Member]
Investment securities
Fair value investments	65.6
Residential | Super Senior | Non-prime
Investment securities
Fair value investments	0.0
Residential | Super Senior | Subprime [Member]
Investment securities
Fair value investments	0.0
Residential | Senior
Investment securities
Fair value investments	67.6
Residential | Senior | Prime [Member]
Investment securities
Fair value investments	67.5
Residential | Senior | Non-prime
Investment securities
Fair value investments	0.1
Residential | Senior | Subprime [Member]
Investment securities
Fair value investments	0.0
Residential | Subordinate
Investment securities
Fair value investments	0.0
Residential | Subordinate | Prime [Member]
Investment securities
Fair value investments	0.0
Residential | Subordinate | Non-prime
Investment securities
Fair value investments	0.0
Residential | Subordinate | Subprime [Member]
Investment securities
Fair value investments	$ 0.0