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Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Schedule of Available-for-sale Securities    
Gross realized gains $ 0.0  
Gross realized losses   $ 0.0
Less than 12 Months 838.6 925.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 5.5 5.4
12 Months or More 16.6 202.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.2 1.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 855.2 1,127.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 5.7 7.2
US Government Agencies Debt Securities [Member]    
Schedule of Available-for-sale Securities    
Less than 12 Months 235.6 185.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2.0 0.8
12 Months or More 0.0 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.0 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 235.6 185.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 2.0 0.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Schedule of Available-for-sale Securities    
Less than 12 Months 434.0 740.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2.4 4.6
12 Months or More 12.3 155.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.1 1.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 446.3 896.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 2.5 6.0
State, county and municipal securities    
Schedule of Available-for-sale Securities    
Less than 12 Months 148.1  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1.0  
12 Months or More 0.0  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.0  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 148.1  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 1.0  
General obligation securities, amortized cost 38.6 80.1
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming    
Schedule of Available-for-sale Securities    
General obligation securities, amortized cost 26.0  
Mortgage-backed Securities, Issued by Private Enterprises [Member]    
Schedule of Available-for-sale Securities    
Less than 12 Months 0.0 0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0.0 0.0
12 Months or More 4.3 46.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.1 0.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 4.3 46.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 0.1 $ 0.4
Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities    
Less than 12 Months 20.9  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0.1  
12 Months or More 0.0  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.0  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 20.9  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss $ 0.1