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Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]      
Gross realized gains $ 0.0   $ 1.1
Gross realized losses   $ 0.0 $ 0.4
Schedule of Available-for-sale Securities      
Less than 12 Months 925.4 1,123.2  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 5.4 22.6  
12 Months or More 202.5 779.0  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1.8 14.5  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 1,127.9 1,902.2  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 7.2 37.1  
US Government Agencies Debt Securities [Member]      
Schedule of Available-for-sale Securities      
Less than 12 Months 185.3 363.1  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0.8 7.9  
12 Months or More 0.0 154.5  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.0 2.3  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 185.3 517.6  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 0.8 10.2  
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]      
Schedule of Available-for-sale Securities      
Less than 12 Months 740.1 735.2  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 4.6 14.5  
12 Months or More 155.9 503.7  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1.4 9.6  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 896.0 1,238.9  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss 6.0 24.1  
State, county and municipal securities      
Schedule of Available-for-sale Securities      
General obligation securities, amortized cost 45.9    
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming      
Schedule of Available-for-sale Securities      
General obligation securities, amortized cost 28.0    
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 46.6 69.4  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0.4 1.8  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 46.6 69.4  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss $ 0.4 1.8  
Corporate Debt Securities [Member]      
Schedule of Available-for-sale Securities      
Less than 12 Months   24.9  
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss   0.2  
12 Months or More   51.4  
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss   0.8  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value   76.3  
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss   $ 1.0