Fair Value (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	$ 4,765.0
Securities carried at fair value with changes recorded in net income	255.6	$ 283.5
Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	4,765.0	3,674.1
Securities carried at fair value with changes recorded in net income	255.6	283.5
Equity Securities AFS	34.5	34.1
Total Assets	5,134.5	4,103.3
Consideration holdback liability	(46.1)	(47.2)
FDIC True-up Liability	(64.3)	(61.9)
Total Liabilities	(225.4)	(177.9)
Recurring [Member] | Non-Qualifying Hedges [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	79.4	94.7
Derivative liabilities at fair value	(72.2)	(68.8)
Recurring [Member] | Qualifying Hedges [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value		16.9
Derivative liabilities at fair value	(42.8)
Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	349.3	200.1
Equity Securities AFS	0.2	0.3
Total Assets	349.5	200.4
Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities AFS	3,958.9	2,988.5
Equity Securities AFS	34.3	33.8
Total Assets	4,072.5	3,133.9
Total Liabilities	(102.3)	(57.3)
Recurring [Member] | Level 2 [Member] | Non-Qualifying Hedges [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	79.3	94.7
Derivative liabilities at fair value	(59.5)	(57.3)
Recurring [Member] | Level 2 [Member] | Qualifying Hedges [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value		16.9
Derivative liabilities at fair value	(42.8)
Recurring [Member] | Level 3 [Member]
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		283.5
Total Assets	712.5	769.0
Consideration holdback liability		(47.2)
FDIC True-up Liability	(64.3)	(61.9)
Total Liabilities	(123.1)	(120.6)
Recurring [Member] | Level 3 [Member] | Non-Qualifying Hedges [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets at fair value	0.1
Derivative liabilities at fair value	$ (12.7)	$ (11.5)