Certain Transfers of Financial Assets - Narrative (Details) loan in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Servicing Assets at Fair Value [Line Items]
Number of loans sold | loan	50
Principal balance of loans sold in secondary market in last four calendar years	$ 12,400.0
Mortgage recourse liability	1.4	$ 1.4
Residential mortgage
Servicing Assets at Fair Value [Line Items]
Sale of loan with unpaid principal	2,600.0	2,800.0
Gain (loss) on sale of mortgage loans	76.4	82.5	$ 69.8
Bank servicing fees	22.2	19.1	15.8
SBA
Servicing Assets at Fair Value [Line Items]
Sale of loan with unpaid principal	47.5	63.1
Bank servicing fees	2.5	2.5	2.3
Gain (loss) on sales of consumer loans	4.2	4.5	3.4
Indirect automobile
Servicing Assets at Fair Value [Line Items]
Sale of loan with unpaid principal	431.2	510.5
Bank servicing fees	9.2	9.2	8.5
Gain (loss) on sales of consumer loans	$ 6.7	$ 9.1	$ 12.4