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Investment Securities (Tables)
 3 Months Ended
Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract]  
Amortized Cost and Approximate Fair Values of Investment Securities
The amortized cost and the approximate fair values of investment securities are summarized as follows:
March 31, 2021Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale:
U.S. Treasury notes$1.0 $— $— $1.0 
State, county, and municipal securities458.0 1.8 (14.7)445.1 
Obligations of U.S. government agencies334.9 0.4 (11.8)323.5 
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations2,866.0 56.3 (20.8)2,901.5 
Private mortgage-backed securities6.5 0.1 — 6.6 
Corporate securities344.6 3.7 (5.9)342.4 
Total$4,011.0 $62.3 $(53.2)$4,020.1 

March 31, 2021Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to-Maturity:
State, county, and municipal securities$74.1 $2.7 $(0.9)$75.9 
U.S agency residential mortgage-backed securities & collateralized mortgage obligations788.2 0.2 (12.8)775.6 
Corporate securities3.9 0.1 — 4.0 
Other investments0.1 — — 0.1 
Total$866.3 $3.0 $(13.7)$855.6 
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 obligations2,830.8 69.3 (2.5)2,897.6 
Private mortgage-backed securities10.9 0.1 (0.1)10.9 
Corporate securities
295.8 6.5 (0.1)302.2 
Other investments
0.2 — — 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 obligations1.0 0.1 — 1.1 
Corporate securities
3.9 0.1 — 4.0 
Other investments0.1 — — 0.1 
Total$51.6 $3.4 $— $55.0 
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 March 31, 2021 and December 31, 2020. There were no held-to-maturity securities in a continuous unrealized loss position at December 31, 2020.
 Less than 12 Months12 Months or MoreTotal
March 31, 2021Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale:      
State, county, and municipal securities$444.8 $(14.7)$— $— $444.8 $(14.7)
Obligations of U.S. government agencies
323.6 (11.8)— — 323.6 (11.8)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations2,750.0 (20.7)11.3 (0.1)2,761.3 (20.8)
Corporate securities342.3 (5.9)— — 342.3 (5.9)
Total$3,860.7 $(53.1)$11.3 $(0.1)$3,872.0 $(53.2)
 Less than 12 Months12 Months or MoreTotal
March 31, 2021Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Held-to-Maturity:      
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations$775.6 $(12.8)$— $— $775.6 $(12.8)
State, county, and municipal securities75.9 (0.9)— — 75.9 (0.9)
Total$851.5 $(13.7)$— $— $851.5 $(13.7)
 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 obligations434.0 (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)
Maturities of Investment Securities As of March 31, 2021, the Company had variable rate mortgage-backed securities with an amortized cost of $235.8 million classified as available-for-sale in the table below. Maturities of mortgage-backed securities have been adjusted to reflect shorter maturities based upon estimated prepayments of principal. All other investment securities maturities are shown at contractual maturity dates.
 Available-for-SaleHeld-to-Maturity
March 31, 2021Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
Within one year$977.9 $948.3 $341.0 $335.7 
After one year but within five years1,124.4 1,335.7 195.6 194.0 
After five years but within ten years1,062.5 1,038.5 147.5 146.9 
After ten years846.2 697.6 182.2 179.0 
Total$4,011.0 $4,020.1 $866.3 $855.6