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Investment Securities (Tables)
 12 Months Ended
Dec. 31, 2020
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 were as follows:
 December 31, 2020
Amortized
Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
 (Dollars in thousands)
Investment securities available-for-sale:    
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 obligation513,259 819 (712)513,366 
Mortgage-backed securities812,384 21,662 (543)833,503 
Total investment securities available-for-sale3,846,733 90,004 (5,622)3,931,115 
Investment securities held-to-maturity:
Mortgage-backed securities22,124 1,281 — 23,405 
Other1,608 — — 1,608 
Total investment securities held-to-maturity23,732 1,281 — 25,013 
Total investment securities$3,870,465 $91,285 $(5,622)$3,956,128 

 December 31, 2019
 Amortized
Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
 (Dollars in thousands)
Investment securities available-for-sale:    
U.S. Treasury$60,457 $3,137 $(39)$63,555 
Agency240,348 7,686 (1,676)246,358 
Corporate149,150 2,217 (14)151,353 
Municipal bonds384,032 13,450 (184)397,298 
Collateralized mortgage obligation9,869 123 (8)9,984 
Mortgage-backed securities494,404 7,603 (2,171)499,836 
Total investment securities available-for-sale1,338,260 34,216 (4,092)1,368,384 
Investment securities held-to-maturity:
Mortgage-backed securities36,114 922 — 37,036 
Other1,724 — — 1,724 
Total investment securities held-to-maturity37,838 922 — 38,760 
Total investment securities$1,376,098 $35,138 $(4,092)$1,407,144 
Schedule of number, fair value and gross unrealized holding losses
The table below shows the 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 unrealized loss position.
 December 31, 2020
 Less than 12 months12 months or LongerTotal
NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses
 (Dollars in thousands)
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 obligation21 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)
 December 31, 2019
 Less than 12 months12 months or LongerTotal
 NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses		NumberFair
Value		Gross
Unrealized
Losses
 (Dollars in thousands)
Investment securities available-for-sale:
U.S. Treasury1 $10,194 $(39)— $— $— 1 $10,194 $(39)
Agency13 102,874 (1,340)9 13,514 (336)22 116,388 (1,676)
Corporate1 1,017 (14)— — — 1 1,017 (14)
Municipal bonds12 30,541 (184)— — — 12 30,541 (184)
Collateralized mortgage obligation— — — 1 603 (8)1 603 (8)
Mortgage-backed securities18 130,014 (1,681)11 26,886 (490)29 156,900 (2,171)
Total investment securities available-for-sale45 $274,640 $(3,258)21 $41,003 $(834)66 $315,643 $(4,092)
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 available-for-sale at December 31, 2020, 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
Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
 (Dollars in thousands)
Investment securities available-for-sale:          
Treasury$— $— $30,153 $32,533 $— $— $— $— $30,153 $32,533 
Agency— — 357,471 365,530 216,904 227,338 92,327 97,518 666,702 690,386 
Corporate146,122 146,471 9,696 9,714 218,964 221,753 37,441 37,370 412,223 415,308 
Municipal bonds9,922 10,525 3,456 3,743 34,623 37,721 1,364,011 1,394,030 1,412,012 1,446,019 
Collateralized mortgage obligation— — 14,938 14,869 219,553 219,509 278,768 278,988 513,259 513,366 
Mortgage-backed securities— — 2,167 2,313 204,924 216,932 605,293 614,258 812,384 833,503 
Total investment securities available-for-sale156,044 156,996 417,881 428,702 894,968 923,253 2,377,840 2,422,164 3,846,733 3,931,115 
Investment securities held-to-maturity:          
Mortgage-backed securities— — — — — — 22,124 23,405 22,124 23,405 
Other— — — — — — 1,608 1,608 1,608 1,608 
Total investment securities held-to-maturity— — — — — — 23,732 25,013 23,732 25,013 
Total investment securities$156,044 $156,996 $417,881 $428,702 $894,968 $923,253 $2,401,572 $2,447,177 $3,870,465 $3,956,128 
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 December 31, 2020:

December 31, 2020
Vintage
20202019201820172016PriorTotal
(Dollars in thousands)
Investment securities available-for-sale:
U.S. Treasury
Aaa - Aa3$— $— $21,852 $10,681 $— $— $32,533 
Agency
Aaa - Aa3359,428 60,943 157,942 9,733 20,711 81,629 690,386 
Corporate debt
A1 - A360,858 — — — 118,631 9,145 188,634 
Baa1 - Baa399,999 69,638 5,034 17,936 5,449 28,618 226,674 
Municipal bonds
Aaa - Aa31,022,544 290,007 32,798 60,595 15,294 23,919 1,445,157 
A1 - A3— — — — — 862 862 
Collateralized mortgage obligations
Aaa - Aa3241,971 107,104 29,890 15,305 105,641 13,455 513,366 
Mortgage-backed securities
Aaa - Aa3424,825 112,447 32,901 144,513 39,975 78,842 833,503 
Total investment securities available-for-sale2,209,625 640,139 280,417 258,763 305,701 236,470 3,931,115 
Investment securities held-to-maturity:
Mortgage-backed securities
Aaa - Aa3— — 6,552 5,372 4,209 5,991 22,124 
Other
Baa1 - Baa3— — 633 — — 975 1,608 
Total investment securities held-to-maturity— — 7,185 5,372 4,209 6,966 23,732 
Total investment securities$2,209,625 $640,139 $287,602 $264,135 $309,910 $243,436 $3,954,847