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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]  
Amortized Cost and Approximate Fair Values of Investment Securities
The amortized cost and approximate fair values of investment securities are summarized as follows:
December 31, 2020Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale
State, county, and municipal securities$462.1 $4.8 $(1.0)$465.9 
Obligations of U.S. government agencies332.9 1.0 (2.0)331.9 
U.S. agency residential mortgage-backed securities &
collateralized mortgage obligations		2,830.8 69.3 (2.5)2,897.6 
Private mortgage-backed securities10.9 0.1 (0.1)10.9 
Corporate Securities295.8 6.5 (0.1)302.2 
Other investments0.2 — — 0.2 
Total$3,932.7 $81.7 $(5.7)$4,008.7 

December 31, 2020Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to Maturity
State, county, and municipal securities$46.6 $3.2 $— $49.8 
U.S agency residential mortgage-backed securities &
collateralized mortgage obligations		1.0 0.1 — 1.1 
Corporate securities3.9 0.1 — 4.0 
Other investments0.1 — — 0.1 
Total$51.6 $3.4 $— $55.0 
December 31, 2019Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale
U.S. Treasury notes$9.0 $— $— $9.0 
State, county, and municipal securities80.1 0.8 — 80.9 
Obligations of U.S. government agencies367.5 0.1 (0.8)366.8 
U.S. agency residential mortgage-backed securities &
collateralized mortgage obligations		2,303.6 19.6 (6.0)2,317.2 
Private mortgage-backed securities47.6 — (0.4)47.2 
Corporate Securities134.5 1.2 — 135.7 
Other investments3.2 — — 3.2 
Total$2,945.5 $21.7 $(7.2)$2,960.0 
December 31, 2019Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to Maturity
State, county, and municipal securities$57.3 $2.1 $— $59.4 
Obligations of U.S. government agencies19.8 — — 19.8 
U.S. agency residential mortgage-backed securities &
collateralized mortgage obligations		1.2 — — 1.2 
Corporate securities13.9 0.1 — 14.0 
Other investments0.1 — — 0.1 
Total$92.3 $2.2 $— $94.5 
Gross Unrealized Losses and Fair Values of Investment Securities
The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of December 31, 2020 and 2019. There were no held-to-maturity securities in a continuous unrealized loss position as of December 31, 2020.
 Less than 12 Months12 Months or MoreTotal
December 31, 2020Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale      
State, county, and municipal securities$148.1 $(1.0)$— $— $148.1 $(1.0)
Obligations of U.S. government agencies235.6 (2.0)— — 235.6 (2.0)
U.S. agency residential mortgage-backed
securities & collateralized mortgage
obligations		434.0 (2.4)12.3 (0.1)446.3 (2.5)
Private mortgage-backed securities— — 4.3 (0.1)4.3 (0.1)
Corporate securities20.9 (0.1)— — 20.9 (0.1)
Total$838.6 $(5.5)$16.6 $(0.2)$855.2 $(5.7)
 Less than 12 Months12 Months or MoreTotal
December 31, 2019Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale      
Obligations of U.S. government agencies$185.3 $(0.8)$— $— $185.3 $(0.8)
U.S. agency residential mortgage-backed
securities & collateralized mortgage
obligations		740.1 (4.6)155.9 (1.4)896.0 (6.0)
Private mortgage-backed securities— — 46.6 (0.4)46.6 (0.4)
Total$925.4 $(5.4)$202.5 $(1.8)$1,127.9 $(7.2)
Maturities of Investment Securities All other investment securities maturities are shown at contractual maturity dates.
 Available-for-SaleHeld-to-Maturity
December 31, 2020Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
Within one year$687.6 $991.4 $8.3 $8.4 
After one year but within five years1,587.7 1,362.6 27.8 29.0 
After five years but within ten years736.2 996.3 14.1 16.2 
After ten years921.2 658.4 1.4 1.4 
Total$3,932.7 $4,008.7 $51.6 $55.0