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Investment Securities (Tables)
 6 Months Ended
Jun. 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 securities were as follows:
 June 30, 2021
(Dollars in thousands)Amortized
 Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
Investment securities available-for-sale:    
U.S. Treasury$109,847 $2,008 $(47)$111,808 
Agency554,342 6,335 (5,510)555,167 
Corporate355,101 5,957 (2,676)358,382 
Municipal bonds1,348,346 36,296 (5,169)1,379,473 
Collateralized mortgage obligations617,596 287 (3,145)614,738 
Mortgage-backed securities1,465,117 11,216 (8,454)1,467,879 
Total investment securities available-for-sale4,450,349 62,099 (25,001)4,487,447 
Investment securities held-to-maturity:
Mortgage-backed securities17,378 887 — 18,265 
Other1,555 — — 1,555 
Total investment securities held-to-maturity18,933 887 — 19,820 
Total investment securities$4,469,282 $62,986 $(25,001)$4,507,267 
 December 31, 2020
(Dollars in thousands)Amortized
Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
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 obligations513,259 819 (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 
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 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 loss position.
 June 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 $38,568 $(47)— $— $— 2 $38,568 $(47)
Agency24 358,145 (5,244)9 9,355 (266)33 367,500 (5,510)
Corporate11 91,496 (2,676)— — — 11 91,496 (2,676)
Municipal bonds68 375,132 (5,169)— — — 68 375,132 (5,169)
Collateralized mortgage obligations35 373,224 (3,145)1 351 — 36 373,575 (3,145)
Mortgage-backed securities.68 838,125 (8,454)— — — 68 838,125 (8,454)
Total investment securities available-for-sale208 $2,074,690 $(24,735)10 $9,706 $(266)218 $2,084,396 $(25,001)

 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 June 30, 2021:
Vintage
(Dollars in thousands)20212020201920182017PriorTotal
Investment securities available-for-sale:
U.S. Treasury
Aaa - Aa3$33,148 $26,266 $20,481 $21,408 $10,505 $— $111,808 
Agency
Aaa - Aa39,572 349,992 55,601 83,752 — 56,250 555,167 
Corporate debt
A1 - A3— 58,502 — — — 60,573 119,075 
Baa1 - Baa336,553 100,494 70,721 — 17,925 13,614 239,307 
Municipal bonds
Aaa - Aa340,975 907,667 297,852 32,830 60,376 36,124 1,375,824 
A1 - A3— — — — — 2,309 2,309 
Baa1 - Baa3— — — — — 1,340 1,340 
Collateralized mortgage obligations
Aaa - Aa3142,912 232,492 91,915 20,786 14,443 112,190 614,738 
Mortgage-backed securities
Aaa - Aa3834,439 437,746 96,261 12,600 40,338 46,495 1,467,879 
Total investment securities available-for-sale1,097,599 2,113,159 632,831 171,376 143,587 328,895 4,487,447 
Investment securities held-to-maturity:
Mortgage-backed securities
Aaa - Aa3— — — 5,029 4,481 7,868 17,378 
Other
Baa1 - Baa3— — — 608 — 947 1,555 
Total investment securities held-to-maturity— — — 5,637 4,481 8,815 18,933 
Total investment securities$1,097,599 $2,113,159 $632,831 $177,013 $148,068 $337,710 $4,506,380 
Schedule of amortized cost and estimated fair value of investment securities available for sale by contractual maturity
The amortized cost and estimated fair value of investment securities at June 30, 2021, by contractual maturity, are shown in the table below.
Due in One Year
or Less		Due after One Year
through Five Years		Due after Five Years
through Ten Years		Due after
Ten Years		Total
(Dollars in thousands)Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
Investment securities available-for-sale:          
U.S. Treasury$— $— $30,135 $31,913 $79,712 $79,895 $— $— $109,847 $111,808 
Agency— — 312,054 314,952 184,783 184,414 57,505 55,801 554,342 555,167 
Corporate54,024 54,074 9,670 9,714 291,407 294,594 — — 355,101 358,382 
Municipal bonds12,830 12,830 4,619 4,918 77,533 80,607 1,253,364 1,281,118 1,348,346 1,379,473 
Collateralized mortgage obligations— — 26,394 26,357 229,519 226,997 361,683 361,384 617,596 614,738 
Mortgage-backed securities— — 2,121 2,252 479,885 486,001 983,111 979,626 1,465,117 1,467,879 
Total investment securities available-for-sale66,854 66,904 384,993 390,106 1,342,839 1,352,508 2,655,663 2,677,929 4,450,349 4,487,447 
Investment securities held-to-maturity:
Mortgage-backed securities— — — — — — 17,378 18,265 17,378 18,265 
Other— — — — — — 1,555 1,555 1,555 1,555 
Total investment securities held-to-maturity— — — — — — 18,933 19,820 18,933 19,820 
Total investment securities$66,854 $66,904 $384,993 $390,106 $1,342,839 $1,352,508 $2,674,596 $2,697,749 $4,469,282 $4,507,267