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Investment Securities (Tables)
 9 Months Ended
Sep. 30, 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:
September 30, 2021Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Available-for-Sale:
U.S. Treasury notes$697.4 $— $(5.6)$691.8 
State, county, and municipal securities444.9 2.1 (7.1)439.9 
Obligations of U.S. government agencies332.3 0.2 (6.6)325.9 
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations2,021.4 25.9 (16.0)2,031.3 
Private mortgage-backed securities83.7 0.1 (0.3)83.5 
Collateralized loan obligations452.4 0.6 — 453.0 
Corporate securities376.7 4.2 (2.7)378.2 
Total$4,408.8 $33.1 $(38.3)$4,403.6 

September 30, 2021Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Estimated
Fair
Value
Held-to-Maturity:
State, county, and municipal securities$69.7 $2.3 $(0.5)$71.5 
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations (1)
1,537.5 20.5 (8.4)1,549.6 
Corporate securities10.9 0.5 — 11.4 
Total$1,618.1 $23.3 $(8.9)$1,632.5 
(1) Amortized costs presented above include $22.3 million of unamortized gains in U.S. agency residential mortgage-backed securities and collateralized mortgage obligations related to the 2021 second quarter transfer of securities from available-for-sale to held-to-maturity.
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 an unrealized loss position as of September 30, 2021 and December 31, 2020. There were no held-to-maturity securities in an unrealized loss position at December 31, 2020.
 Less than 12 Months12 Months or MoreTotal
September 30, 2021Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses		Fair
Value		Gross
Unrealized
Losses
Available-for-Sale:      
U.S. Treasury notes$691.8 $(5.6)$— $— $691.8 $(5.6)
State, county, and municipal securities287.7 (7.1)— — 287.7 (7.1)
Obligations of U.S. government agencies
301.2 (6.6)— — 301.2 (6.6)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations964.9 (16.0)— — 964.9 (16.0)
Private mortgage-backed securities54.3 (0.3)— — 54.3 (0.3)
Corporate securities158.7 (2.6)21.2 (0.1)179.9 (2.7)
Total$2,458.6 $(38.2)$21.2 $(0.1)$2,479.8 $(38.3)
 Less than 12 Months12 Months or MoreTotal
September 30, 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$772.3 $(8.4)$— $— $772.3 $(8.4)
State, county, and municipal securities29.0 (0.5)— — 29.0 (0.5)
Total$801.3 $(8.9)$— $— $801.3 $(8.9)

 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 , the Company had variable rate mortgage-backed and corporate securities which had an amortized cost of $540.2 million and were classified as available-for-sale as of September 30, 2021. 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
September 30, 2021Amortized
Cost		Estimated
Fair Value		Amortized
Cost		Estimated
Fair Value
Within one year$625.0 $722.7 $662.2 $667.4 
After one year but within five years1,514.8 1,621.1 365.7 369.7 
After five years but within ten years1,295.3 1,161.6 263.6 266.8 
After ten years973.7 898.2 326.6 328.6 
Total$4,408.8 $4,403.6 $1,618.1 $1,632.5