Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Net accumulated other comprehensive income:
Net unrealized gain (loss) on investment securities available-for-sale	$ (33.5)	$ (13.3)
Net unrealized gain (loss) on derivatives	0.0	1.3
Net actuarial gain (loss) on defined benefit post-retirement benefit plans	1.2	0.0
Net accumulated other comprehensive gain (loss)	$ (32.3)	$ (12.0)