Fair Value (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | $ 6,308.8 | $ 5,931.3 |
U.S. government agency obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 10 | 34.5 |
U.S. Treasury securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 728.8 | 251.5 |
Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 6,308.8 | 5,931.3 |
Securities carried at fair value with changes recorded in net income | 46.4 | 44.6 |
Total derivative assets at fair value — non-qualifying hedges | 200.8 | 91.1 |
Total Assets | 6,561 | 6,095.8 |
FDIC True-up liability | (67.8) | (66.9) |
Total Liabilities | (227.4) | (146.6) |
Recurring | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative liabilities at fair value — non-qualifying hedges | (152.3) | (79.4) |
Recurring | Derivatives designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 5 | 28.8 |
Total derivative liabilities at fair value — non-qualifying hedges | (7.3) | (0.3) |
Recurring | Interest rate contracts | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 186.4 | 87.8 |
Total derivative liabilities at fair value — non-qualifying hedges | (148) | (59.7) |
Recurring | Interest rate contracts | Fair Value Hedges | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 5 | 1.9 |
Total derivative liabilities at fair value — non-qualifying hedges | | 0 |
Recurring | Other Derivative Non qualifying Hedges | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 14.4 | 3.3 |
Total derivative liabilities at fair value — non-qualifying hedges | (4.3) | (19.7) |
Recurring | Foreign Currency Forward Contract | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 26.9 |
Total derivative liabilities at fair value — non-qualifying hedges | (7.3) | (0.3) |
Recurring | U.S. government agency obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 4,882.5 | 5,225.2 |
Recurring | U.S. Treasury securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 728.8 | 251.5 |
Recurring | Other Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 697.5 | 454.6 |
Recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 572.9 | 53.9 |
Securities carried at fair value with changes recorded in net income | 0.1 | 0.1 |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total Assets | 573 | 54 |
FDIC True-up liability | 0 | 0 |
Total Liabilities | 0 | 0 |
Recurring | Level 1 | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 1 | Derivatives designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 1 | Interest rate contracts | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 1 | Interest rate contracts | Fair Value Hedges | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | | 0 |
Recurring | Level 1 | Other Derivative Non qualifying Hedges | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 1 | Foreign Currency Forward Contract | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 1 | U.S. government agency obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 0 | 0 |
Recurring | Level 1 | U.S. Treasury securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 572.9 | 53.9 |
Recurring | Level 1 | Other Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 0 | 0 |
Recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 5,668.6 | 5,811.5 |
Securities carried at fair value with changes recorded in net income | 46.3 | 44.5 |
Total derivative assets at fair value — non-qualifying hedges | 200.4 | 90.7 |
Total Assets | 5,920.3 | 5,975.5 |
FDIC True-up liability | 0 | 0 |
Total Liabilities | (159.5) | (79.7) |
Recurring | Level 2 | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative liabilities at fair value — non-qualifying hedges | (152.2) | (79.4) |
Recurring | Level 2 | Derivatives designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 5 | 28.8 |
Total derivative liabilities at fair value — non-qualifying hedges | (7.3) | (0.3) |
Recurring | Level 2 | Interest rate contracts | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 186.1 | 87.6 |
Total derivative liabilities at fair value — non-qualifying hedges | (148) | (59.7) |
Recurring | Level 2 | Interest rate contracts | Fair Value Hedges | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 5 | 1.9 |
Total derivative liabilities at fair value — non-qualifying hedges | | 0 |
Recurring | Level 2 | Other Derivative Non qualifying Hedges | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 14.3 | 3.1 |
Total derivative liabilities at fair value — non-qualifying hedges | (4.2) | (19.7) |
Recurring | Level 2 | Foreign Currency Forward Contract | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 26.9 |
Total derivative liabilities at fair value — non-qualifying hedges | (7.3) | (0.3) |
Recurring | Level 2 | U.S. government agency obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 4,882.5 | 5,225.2 |
Recurring | Level 2 | U.S. Treasury securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 155.9 | 197.6 |
Recurring | Level 2 | Other Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 630.2 | 388.7 |
Recurring | Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 67.3 | 65.9 |
Securities carried at fair value with changes recorded in net income | 0 | 0 |
Total derivative assets at fair value — non-qualifying hedges | 0.4 | 0.4 |
Total Assets | 67.7 | 66.3 |
FDIC True-up liability | (67.8) | (66.9) |
Total Liabilities | (67.9) | (66.9) |
Recurring | Level 3 | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative liabilities at fair value — non-qualifying hedges | (0.1) | 0 |
Recurring | Level 3 | Derivatives designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 3 | Interest rate contracts | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0.3 | 0.2 |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 3 | Interest rate contracts | Fair Value Hedges | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | | 0 |
Recurring | Level 3 | Other Derivative Non qualifying Hedges | Derivatives not designated as hedging instruments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0.1 | 0.2 |
Total derivative liabilities at fair value — non-qualifying hedges | (0.1) | 0 |
Recurring | Level 3 | Foreign Currency Forward Contract | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total derivative assets at fair value — non-qualifying hedges | 0 | 0 |
Total derivative liabilities at fair value — non-qualifying hedges | 0 | 0 |
Recurring | Level 3 | U.S. government agency obligations | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 0 | 0 |
Recurring | Level 3 | U.S. Treasury securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | 0 | 0 |
Recurring | Level 3 | Other Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Debt securities, Fair Value | $ 67.3 | $ 65.9 |