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Debt Securities (Tables)
 12 Months Ended
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]  
Amortized cost, gross unrealized gains and losses and the fair value of debt securities
The following tables present the amortized cost, the gross unrealized gains and losses and the fair value of the Company’s debt securities:

 December 31, 2020
 Amortized
Cost		Gross UnrealizedFair
Value
(Dollars in thousands)GainsLosses
Available-for-sale
U.S. government and federal agency$38,568 287 (267)38,588 
U.S. government sponsored enterprises9,747 34 — 9,781 
State and local governments1,321,763 94,974 (54)1,416,683 
Corporate bonds336,867 12,239 (8)349,098 
Residential mortgage-backed securities2,261,463 27,631 (4)2,289,090 
Commercial mortgage-backed securities1,177,458 57,575 (459)1,234,574 
Total available-for-sale5,145,866 192,740 (792)5,337,814 
Held-to-maturity
State and local governments189,836 13,380 — 203,216 
Total held-to-maturity189,836 13,380 — 203,216 
Total debt securities$5,335,702 206,120 (792)5,541,030 

 December 31, 2019
 Amortized
Cost		Gross UnrealizedFair
Value
(Dollars in thousands)GainsLosses
Available-for-sale
U.S. government and federal agency$20,061 48 (65)20,044 
U.S. government sponsored enterprises42,724 953 — 43,677 
State and local governments679,784 22,694 (80)702,398 
Corporate bonds155,665 1,938 (1)157,602 
Residential mortgage-backed securities731,766 7,507 (549)738,724 
Commercial mortgage-backed securities891,374 22,825 (1,392)912,807 
Total available-for-sale2,521,374 55,965 (2,087)2,575,252 
Held-to-maturity
State and local governments224,611 9,785 — 234,396 
Total held-to-maturity224,611 9,785 — 234,396 
Total debt securities$2,745,985 65,750 (2,087)2,809,648 
Amortized cost and fair value of debt securities by contractual maturity
The following table presents the amortized cost and fair value of available-for-sale and held-to-maturity debt securities by contractual maturity at December 31, 2020. Actual maturities may differ from expected or contractual maturities since some issuers have the right to prepay obligations with or without prepayment penalties.

 December 31, 2020
 Available-for-SaleHeld-to-Maturity
(Dollars in thousands)Amortized CostFair ValueAmortized CostFair Value
Due within one year$128,263 129,346 484 489 
Due after one year through five years246,473 258,040 22,032 23,625 
Due after five years through ten years266,248 280,021 67,678 73,367 
Due after ten years1,065,961 1,146,743 99,642 105,735 
1,706,945 1,814,150 189,836 203,216 
Mortgage-backed securities 1
3,438,921 3,523,664 — — 
Total$5,145,866 5,337,814 189,836 203,216 
______________________________
1 Mortgage-backed securities, which have prepayment provisions, are not assigned to maturity categories due to fluctuations in their prepayment speeds.
Proceeds from sales and calls of debt securities and associated gains and losses
Proceeds from sales and calls of debt securities and the associated gains and losses that have been included in earnings are listed below:

 Years ended
(Dollars in thousands)December 31,
2020		December 31,
2019		December 31,
2018
Available-for-sale
Proceeds from sales and calls of debt securities$240,521 928,710 265,587 
Gross realized gains 1
1,400 18,936 443 
Gross realized losses 1
(262)(4,513)(455)
Held-to-maturity
Proceeds from calls of debt securities32,735 58,750 79,000 
Gross realized gains 1
1 2 101 
Gross realized losses 1
— (10)(1,202)
______________________________
1 The gain or loss on the sale or call of each debt security is determined by the specific identification method.
Summary of debt securities with an unrealized loss position
 December 31, 2020
 NumberLess than 12 Months12 Months or MoreTotal
(Dollars in thousands)of
Securities		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss
Available-for-sale
U.S. government and federal agency
18 $13,814 (258)726 (9)14,540 (267)
State and local governments6 3,121 (54)— — 3,121 (54)
Corporate bonds3 5,500 (8)— — 5,500 (8)
Residential mortgage-backed securities14 2,354 (4)27 — 2,381 (4)
Commercial mortgage-backed securities5 120,741 (459)— — 120,741 (459)
Total available-for-sale
46 $145,530 (783)753 (9)146,283 (792)

 December 31, 2019
 NumberLess than 12 Months12 Months or MoreTotal
(Dollars in thousands)of
Securities		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss
Available-for-sale
U.S. government and federal agency
20 $464 — 9,902 (65)10,366 (65)
State and local governments12 19,044 (80)— — 19,044 (80)
Corporate bonds2 7,378 (1)— — 7,378 (1)
Residential mortgage-backed securities35 85,562 (234)29,038 (315)114,600 (549)
Commercial mortgage-backed securities19 177,051 (1,293)7,697 (99)184,748 (1,392)
Total available-for-sale
88 $289,499 (1,608)46,637 (479)336,136 (2,087)
Credit quality indictors for HTM debt securities
Allowance for Credit Losses - Held-To-Maturity Debt Securities
The Company measured expected credit losses on held-to-maturity debt securities on a collective basis by major security type and NRSRO credit ratings, which is the Company’s primary credit quality indicator for state and local government securities. The estimate of expected credit losses considered historical credit loss information that was adjusted for current conditions as well as reasonable and supportable forecasts. The following table summarizes the amortized cost of held-to-maturity debt securities aggregated by NRSRO credit rating:

(Dollars in thousands)December 31,
2020		December 31,
2019
Held-to-maturity
S&P: AAA / Moody’s: Aaa
$39,022 65,217 
S&P: AA+, AA, AA- / Moody’s: Aa1, Aa2, Aa3
123,664 130,316 
S&P: A+, A, A- / Moody’s: A1, A2, A3
27,150 28,689 
Not rated by either entity
— 389 
Total held-to-maturity
$189,836 224,611