Fair Value (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	$ 4,973.6
Securities carried at fair value with changes recorded in net income	247.7	$ 283.5
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	4,973.6	3,674.1
Securities carried at fair value with changes recorded in net income	247.7	283.5
Equity Securities AFS	34.7	34.1
Total Assets	5,327.5	4,103.3
Consideration holdback liability	(46.0)	(47.2)
FDIC True-up Liability	(64.6)	(61.9)
Total Liabilities	(190.6)	(177.9)
Recurring | Non-Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	70.4	94.7
Derivative liabilities at fair value	(72.8)	(68.8)
Recurring | Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	1.1	16.9
Derivative liabilities at fair value	(7.2)
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	398.6	200.1
Securities carried at fair value with changes recorded in net income	0.0	0.0
Equity Securities AFS	0.2	0.3
Total Assets	398.8	200.4
Consideration holdback liability	0.0	0.0
FDIC True-up Liability	0.0	0.0
Total Liabilities	0.0	0.0
Recurring | Level 1 | Non-Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	0.0	0.0
Derivative liabilities at fair value	0.0	0.0
Recurring | Level 1 | Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	0.0	0.0
Derivative liabilities at fair value	0.0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	4,164.0	2,988.5
Securities carried at fair value with changes recorded in net income	0.0	0.0
Equity Securities AFS	34.5	33.8
Total Assets	4,269.9	3,133.9
Consideration holdback liability	0.0	0.0
FDIC True-up Liability	0.0	0.0
Total Liabilities	(66.2)	(57.3)
Recurring | Level 2 | Non-Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	70.3	94.7
Derivative liabilities at fair value	(59.0)	(57.3)
Recurring | Level 2 | Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	1.1	16.9
Derivative liabilities at fair value	(7.2)
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS		485.5
Securities carried at fair value with changes recorded in net income	247.7	283.5
Equity Securities AFS	0.0	0.0
Total Assets	658.8	769.0
Consideration holdback liability		(47.2)
FDIC True-up Liability	(64.6)	(61.9)
Total Liabilities	(124.4)	(120.6)
Recurring | Level 3 | Non-Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	0.1	0.0
Derivative liabilities at fair value	(13.8)	(11.5)
Recurring | Level 3 | Qualifying Hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	0.0	$ 0.0
Derivative liabilities at fair value	$ 0.0