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Debt Securities (Tables)
 12 Months Ended
Dec. 31, 2021
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, 2021
 Amortized
Cost		Gross Unrealized GainsGross Unrealized LossesFair
Value
(Dollars in thousands)
Available-for-sale
U.S. government and federal agency$1,356,171 174 (9,596)1,346,749 
U.S. government sponsored enterprises241,687 2 (996)240,693 
State and local governments461,414 27,567 (123)488,858 
Corporate bonds175,697 5,072 (17)180,752 
Residential mortgage-backed securities5,744,505 9,420 (54,266)5,699,659 
Commercial mortgage-backed securities1,195,949 25,882 (7,693)1,214,138 
Total available-for-sale9,175,423 68,117 (72,691)9,170,849 
Held-to-maturity
State and local governments1,199,164 22,878 (1,159)1,220,883 
Total held-to-maturity1,199,164 22,878 (1,159)1,220,883 
Total debt securities$10,374,587 90,995 (73,850)10,391,732 
 December 31, 2020
 Amortized
Cost		Gross Unrealized GainsGross Unrealized LossesFair
Value
(Dollars in thousands)
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 
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, 2021. 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, 2021
 Available-for-SaleHeld-to-Maturity
(Dollars in thousands)Amortized CostFair ValueAmortized CostFair Value
Due within one year$54,460 54,824 1,432 1,445 
Due after one year through five years1,143,487 1,146,529 29,286 30,932 
Due after five years through ten years815,328 816,598 91,688 96,253 
Due after ten years221,694 239,101 1,076,758 1,092,253 
2,234,969 2,257,052 1,199,164 1,220,883 
Mortgage-backed securities 1
6,940,454 6,913,797 — — 
Total$9,175,423 9,170,849 1,199,164 1,220,883 
______________________________
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,
2021		December 31,
2020		December 31,
2019
Available-for-sale
Proceeds from sales and calls of debt securities$188,431 240,521 928,710 
Gross realized gains 1
984 1,400 18,936 
Gross realized losses 1
(194)(262)(4,513)
Held-to-maturity
Proceeds from calls of debt securities48,475 32,735 58,750 
Gross realized gains 1
3 1 2 
Gross realized losses 1
(1,431)— (10)
______________________________
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
The following table summarizes available-for-sale debt securities that were in an unrealized loss position for which an ACL has not been recorded, based on the length of time the individual securities have been in an unrealized loss position. The number of available-for-sale debt securities in an unrealized position is also disclosed.
 
 December 31, 2021
 Number
of
Securities		Less than 12 Months12 Months or MoreTotal
(Dollars in thousands)Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss
Available-for-sale
U.S. government and federal agency
50 $1,329,399 (9,344)5,457 (252)1,334,856 (9,596)
U.S. government sponsored enterprises
11 239,928 (996)— — 239,928 (996)
State and local governments10 11,080 (83)1,760 (40)12,840 (123)
Corporate bonds3 12,483 (17)— — 12,483 (17)
Residential mortgage-backed securities151 5,335,632 (53,434)53,045 (832)5,388,677 (54,266)
Commercial mortgage-backed securities38 302,784 (3,316)126,798 (4,377)429,582 (7,693)
Total available-for-sale
263 $7,231,306 (67,190)187,060 (5,501)7,418,366 (72,691)
 December 31, 2020
 Number
of
Securities		Less than 12 Months12 Months or MoreTotal
(Dollars in thousands)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)
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,
2021		December 31,
2020
Held-to-maturity
S&P: AAA / Moody’s: Aaa
$316,899 39,022 
S&P: AA+, AA, AA- / Moody’s: Aa1, Aa2, Aa3
841,616 123,664 
S&P: A+, A, A- / Moody’s: A1, A2, A3
39,078 27,150 
Not rated by either entity
1,571 — 
Total held-to-maturity
$1,199,164 189,836