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Investment Securities (Tables)
 3 Months Ended
Mar. 31, 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:
 March 31, 2021
 Amortized
 Cost		Gross Unrealized
Gain		Gross Unrealized
Loss		Estimated
Fair Value
 (Dollars in thousands)
Investment securities available-for-sale:    
U.S. Treasury$30,144 $1,919 $— $32,063 
Agency660,582 12,956 (10,379)663,159 
Corporate360,617 2,128 (4,911)357,834 
Municipal bonds1,432,169 22,287 (32,394)1,422,062 
Collateralized mortgage obligations514,250 431 (5,710)508,971 
Mortgage-backed securities880,903 9,309 (16,964)873,248 
Total investment securities available-for-sale3,878,665 49,030 (70,358)3,857,337 
Investment securities held-to-maturity:
Mortgage-backed securities20,353 1,073 — 21,426 
Other1,578 — — 1,578 
Total investment securities held-to-maturity21,931 1,073 — 23,004 
Total investment securities$3,900,596 $50,103 $(70,358)$3,880,341 
 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 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.
 March 31, 2021
 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:
Agency28 $389,387 $(10,094)9 $9,947 $(285)37 $399,334 $(10,379)
Corporate18 152,134 (4,911)— — — 18 152,134 (4,911)
Municipal bonds173 921,332 (32,394)— — — 173 921,332 (32,394)
Collateralized mortgage obligations32 309,975 (5,708)1 388 (2)33 310,363 (5,710)
Mortgage-backed securities.70 671,400 (16,964)— — — 70 671,400 (16,964)
Total investment securities available-for-sale321 $2,444,228 $(70,071)10 $10,335 $(287)331 $2,454,563 $(70,358)

 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 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 $(5,622)
Schedule of amortized cost and estimated fair value of investment securities available for sale by contractual maturity
The amortized cost and estimated fair value of investment securities at March 31, 2021, by contractual maturity, are shown in the table below.
Due in One Year
or Less		Due after One Year
through Five Years		Due after Five Years
through Ten Years		Due after
Ten Years		Total
Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value		Amortized
Cost		Fair
Value
 (Dollars in thousands)
Investment securities available-for-sale:          
U.S. Treasury$— $— $30,144 $32,063 $— $— $— $— $30,144 $32,063 
Agency— — 322,038 325,584 251,286 252,084 87,258 85,491 660,582 663,159 
Corporate94,711 94,917 9,683 9,645 218,847 218,536 37,376 34,736 360,617 357,834 
Municipal bonds11,286 11,286 4,622 4,912 36,182 38,652 1,380,079 1,367,212 1,432,169 1,422,062 
Collateralized mortgage obligations— — 27,394 27,373 211,472 207,662 275,384 273,936 514,250 508,971 
Mortgage-backed securities— — 2,144 2,283 190,923 195,067 687,836 675,898 880,903 873,248 
Total investment securities available-for-sale105,997 106,203 396,025 401,860 908,710 912,001 2,467,933 2,437,273 3,878,665 3,857,337 
Investment securities held-to-maturity:
Mortgage-backed securities— — — — — — 20,353 21,426 20,353 21,426 
Other— — — — — — 1,578 1,578 1,578 1,578 
Total investment securities held-to-maturity— — — — — — 21,931 23,004 21,931 23,004 
Total investment securities$105,997 $106,203 $396,025 $401,860 $908,710 $912,001 $2,489,864 $2,460,277 $3,900,596 $3,880,341 
Schedule of investment securities by external credit rating
The following table summarizes the Company’s investment securities portfolio by Moody’s external rating equivalent and by vintage as of March 31, 2021:
Vintage
20212020201920182017PriorTotal
(Dollars in thousands)
Investment securities available-for-sale:
U.S. Treasury
Aaa - Aa3$— $— $— $21,510 $10,553 $— $32,063 
Agency
Aaa - Aa3— 352,414 73,090 151,364 9,075 77,216 663,159 
Corporate debt
A1 - A3— 56,760 — — — 93,295 150,055 
Baa1 - Baa3— 98,707 69,231 — 17,919 21,922 207,779 
Municipal bonds
Aaa - Aa311,286 994,512 285,174 32,304 59,458 35,689 1,418,423 
A1 - A3— — — — — 2,313 2,313 
Baa1 - Baa3— — — — — 1,326 1,326 
Collateralized mortgage obligations
Aaa - Aa317,334 241,843 93,697 25,311 14,634 116,152 508,971 
Mortgage-backed securities
Aaa - Aa3185,205 458,327 109,257 13,155 47,529 59,775 873,248 
Total investment securities available-for-sale213,825 2,202,563 630,449 243,644 159,168 407,688 3,857,337 
Investment securities held-to-maturity:
Mortgage-backed securities
Aaa - Aa3— — — 5,822 5,186 9,345 20,353 
Other
Baa1 - Baa3— — — 617 — 961 1,578 
Total investment securities held-to-maturity— — — 6,439 5,186 10,306 21,931 
Total investment securities$213,825 $2,202,563 $630,449 $250,083 $164,354 $417,994 $3,879,268