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Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Jun. 07, 2021
Dec. 31, 2021
Dec. 31, 2020
Schedule of Available for Sale and Held-to-Maturity Securities      
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost   $ 647.4  
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   4,859.3 $ 3,932.7
Gross Unrealized Gains   20.6 81.7
Gross Unrealized Losses   (59.4) (5.7)
Estimated Fair Value   4,820.5 4,008.7
Held-to-Maturity:      
Amortized Cost   1,687.6 51.6
Gross Unrealized Gains   15.6 3.4
Gross Unrealized Losses   35.7 0.0
Estimated Fair Value   1,667.5 55.0
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value   742.8  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost   696.4  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value   687.2  
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost   1,482.4  
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value   1,577.4  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost   374.5  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value   363.6  
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost   1,313.4  
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value   1,165.6  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost   275.6  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value   276.2  
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost   1,416.1  
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value   1,334.7  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost   341.1  
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value   340.5  
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss $ 646.7    
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense 672.2    
US Treasury notes      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   697.6  
Gross Unrealized Gains   0.0  
Gross Unrealized Losses   (12.9)  
Estimated Fair Value   684.7  
State, county and municipal securities      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   434.7 462.1
Gross Unrealized Gains   2.1 4.8
Gross Unrealized Losses   (9.3) (1.0)
Estimated Fair Value   427.5 465.9
Held-to-Maturity:      
Amortized Cost   67.6 46.6
Gross Unrealized Gains   2.0 3.2
Gross Unrealized Losses   0.4 0.0
Estimated Fair Value   69.2 49.8
US Government Agencies Debt Securities [Member]      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   356.0 332.9
Gross Unrealized Gains   0.1 1.0
Gross Unrealized Losses   (9.2) (2.0)
Estimated Fair Value   346.9 331.9
Corporate Debt Securities [Member]      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   271.1 295.8
Gross Unrealized Gains   3.0 6.5
Gross Unrealized Losses   (3.6) (0.1)
Estimated Fair Value   270.5 302.2
Held-to-Maturity:      
Amortized Cost   11.0 3.9
Gross Unrealized Gains   0.4 0.1
Gross Unrealized Losses   0.0 0.0
Estimated Fair Value   11.4 4.0
Debt and Equity Securities, Unrealized Gain (Loss)   20.1  
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss 17.3    
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense 17.7    
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   2,027.3 2,830.8
Gross Unrealized Gains   14.1 69.3
Gross Unrealized Losses   (23.3) (2.5)
Estimated Fair Value   2,018.1 2,897.6
Held-to-Maturity:      
Amortized Cost   1,609.0 1.0
Gross Unrealized Gains   13.2 0.1
Gross Unrealized Losses   35.3 0.0
Estimated Fair Value   1,586.9 1.1
Debt Securities, Available-for-Sale and Held-to-Maturity, Amortized Cost, before Allowance for Credit Loss 629.4    
Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Allowance for Credit Loss, Expense $ 654.5    
Mortgage-backed Securities, Issued by Private Enterprises [Member]      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   174.4 10.9
Gross Unrealized Gains   0.1 0.1
Gross Unrealized Losses   (1.1) (0.1)
Estimated Fair Value   173.4 10.9
Other Debt Obligations [Member]      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost     0.2
Gross Unrealized Gains     0.0
Gross Unrealized Losses     0.0
Estimated Fair Value     0.2
Other Investments      
Held-to-Maturity:      
Amortized Cost     0.1
Gross Unrealized Gains     0.0
Gross Unrealized Losses     0.0
Estimated Fair Value     $ 0.1
Collateralized Loan Obligations      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   898.2  
Gross Unrealized Gains   1.2  
Gross Unrealized Losses   0.0  
Estimated Fair Value   899.4  
US States And Political Subdivisions Member, General Obligation Securities      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   55.9  
Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming | State, county and municipal securities      
Available-for-Sale:      
Debt Securities, Available-for-sale, Amortized Cost   40.7  
Callable Within One Year [Member]      
Schedule of Available for Sale and Held-to-Maturity Securities      
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis   236.1  
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value   235.7  
Held-to-Maturity:      
Investment Securities, Callable Structured, Amortized Cost Basis   $ 0.0