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Investment Securities (Tables)
 9 Months Ended
Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]  
Schedule of amortized cost and estimated fair value of securities
The amortized cost and estimated fair value of investment securities available-for-sale were as follows:
(Dollars in thousands)Amortized
 Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
Investment securities available-for-sale:
September 30, 2021    
U.S. Treasury$77,803 $1,665 $(126)$79,342 
Agency531,867 5,252 (7,026)530,093 
Corporate383,910 5,381 (2,900)386,391 
Municipal bonds1,237,923 27,585 (9,642)1,255,866 
Collateralized mortgage obligations665,851 1,109 (4,729)662,231 
Mortgage-backed securities1,807,758 6,194 (18,060)1,795,892 
Total investment securities available-for-sale$4,705,112 $47,186 $(42,483)$4,709,815 
December 31, 2020
U.S. Treasury$30,153 $2,380 $— $32,533 
Agency666,702 24,292 (608)690,386 
Corporate412,223 3,591 (506)415,308 
Municipal bonds1,412,012 37,260 (3,253)1,446,019 
Collateralized mortgage obligations513,259 819 (712)513,366 
Mortgage-backed securities812,384 21,662 (543)833,503 
Total investment securities available-for-sale$3,846,733 $90,004 $(5,622)$3,931,115 

The carrying amount and estimated fair value of investment securities held-to-maturity were as follows:
(Dollars in thousands)Amortized
 Cost		Allowance for Credit LossesNet Carrying AmountGross Unrecognized
Gain		Gross Unrecognized
Loss		Estimated
Fair Value
Investment securities held-to-maturity:
September 30, 2021
Municipal bonds$154,399 $(11)$154,388 $— $(2,225)$152,163 
Mortgage-backed securities14,656 — 14,656 803 — 15,459 
Other1,532 — 1,532 — — 1,532 
Total investment securities held-to-maturity$170,587 $(11)$170,576 $803 $(2,225)$169,154 
December 31, 2020
Mortgage-backed securities$22,124 $— $22,124 $1,281 $— $23,405 
Other1,608 — 1,608 — — 1,608 
Total investment securities held-to-maturity$23,732 $— $23,732 $1,281 $— $25,013 
Schedule of number, fair value and gross unrealized holding losses of the Company's investment securities by investment category and length of time that the securities have been in a continuous loss position
The table below summarizes the number, fair value, and gross unrealized holding losses of the Company’s investment securities available-for-sale in an unrealized loss position for which an allowance for credit losses has not been recorded as of the dates indicated, aggregated by investment category and length of time in a continuous loss position.
 September 30, 2021
 Less than 12 Months12 Months or LongerTotal
(Dollars in thousands)NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses
Investment securities available-for-sale:
U.S. Treasury2 $32,626 $(126)— $— $— 2 $32,626 $(126)
Agency24 339,270 (5,365)12 64,991 (1,661)36 404,261 (7,026)
Corporate8 88,666 (1,916)1 20,346 (984)9 109,012 (2,900)
Municipal bonds108 651,169 (9,102)2 11,068 (540)110 662,237 (9,642)
Collateralized mortgage obligations30 302,104 (4,297)5 44,615 (432)35 346,719 (4,729)
Mortgage-backed securities.113 1,570,327 (17,961)1 7,395 (99)114 1,577,722 (18,060)
Total investment securities available-for-sale285 $2,984,162 $(38,767)21 $148,415 $(3,716)306 $3,132,577 $(42,483)
 December 31, 2020
 Less than 12 Months12 Months or LongerTotal
(Dollars in thousands)NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses
Investment securities available-for-sale:
Agency4 $74,194 $(307)9 $10,434 $(301)13 $84,628 $(608)
Corporate9 71,226 (506)— — — 9 71,226 (506)
Municipal bonds56 312,894 (3,253)— — — 56 312,894 (3,253)
Collateralized mortgage obligations21 215,603 (710)1 431 (2)22 216,034 (712)
Mortgage-backed securities16 139,071 (543)— — — 16 139,071 (543)
Total investment securities available-for-sale106 $812,988 $(5,319)10 $10,865 $(303)116 $823,853 $(5,622)
Schedule of investment securities by external credit rating The following table summarizes the Company’s investment securities portfolio by Moody’s external rating equivalent and by vintage as of September 30, 2021:
Vintage
(Dollars in thousands)20212020201920182017PriorTotal
Investment securities available-for-sale at fair value:
U.S. Treasury
Aaa - Aa3$32,973 $14,634 $— $21,274 $10,461 $— $79,342 
Agency
Aaa - Aa324,393 324,241 44,487 76,415 — 46,390 515,926 
Baa1 - Baa3— — — 5,715 — 8,452 14,167 
Corporate debt
A1 - A379,252 57,799 — — 17,864 10,029 164,944 
Baa1 - Baa336,473 100,614 70,704 — — 13,656 221,447 
Municipal bonds
Aaa - Aa3118,941 725,524 294,323 32,429 50,536 33,264 1,255,017 
A1 - A3— — — — — 849 849 
Collateralized mortgage obligations
Aaa - Aa3212,687 224,252 86,604 18,523 14,199 105,966 662,231 
Mortgage-backed securities
Aaa - Aa31,250,191 391,394 101,935 6,891 25,280 20,201 1,795,892 
Total investment securities available-for-sale1,754,910 1,838,458 598,053 161,247 118,340 238,807 4,709,815 
Investment securities held-to-maturity at amortized cost:
Municipal bonds
Aaa - Aa3— 154,399 — — — — 154,399 
Mortgage-backed securities
Aaa - Aa3— — — 4,206 3,767 6,683 14,656 
Other
Baa1 - Baa3— — — 600 — 932 1,532 
Total investment securities held-to-maturity— 154,399 — 4,806 3,767 7,615 170,587 
Total investment securities$1,754,910 $1,992,857 $598,053 $166,053 $122,107 $246,422 $4,880,402 
Schedule of amortized cost and estimated fair value of investment securities available for sale by contractual maturity
The amortized cost and estimated fair value of investment securities at September 30, 2021, by contractual maturity, are shown in the table below.
Due in One Year
or Less		Due after One Year
through Five Years		Due after Five Years
through Ten Years		Due after
Ten Years		Total
(Dollars in thousands)Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
Investment securities available-for-sale:          
U.S. Treasury$— $— $30,126 $31,734 $47,677 $47,608 $— $— $77,803 $79,342 
Agency— — 352,023 353,184 135,567 134,426 44,277 42,483 531,867 530,093 
Corporate10,021 10,029 21,178 21,227 352,711 355,135 — — 383,910 386,391 
Municipal bonds— — 3,528 3,731 95,420 96,849 1,138,975 1,155,286 1,237,923 1,255,866 
Collateralized mortgage obligations— — 41,692 41,631 226,958 223,612 397,201 396,988 665,851 662,231 
Mortgage-backed securities— — 12,098 12,704 709,280 709,155 1,086,380 1,074,033 1,807,758 1,795,892 
Total investment securities available-for-sale10,021 10,029 460,645 464,211 1,567,613 1,566,785 2,666,833 2,668,790 4,705,112 4,709,815 
Investment securities held-to-maturity:
Municipal bonds— — — — — — 154,399 152,163 154,399 152,163 
Mortgage-backed securities— — — — — — 14,656 15,459 14,656 15,459 
Other— — — — — — 1,532 1,532 1,532 1,532 
Total investment securities held-to-maturity— — — — — — 170,587 169,154 170,587 169,154 
Total investment securities$10,021 $10,029 $460,645 $464,211 $1,567,613 $1,566,785 $2,837,420 $2,837,944 $4,875,699 $4,878,969