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Investment securities - Summary of Amortized Cost of Securities and Fair Values (Details) - USD ($) | Sep. 30, 2020 | Dec. 31, 2019
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Debt Securities, Available-for-sale [Abstract] |  |
Amortized cost | $ 1,129,053,000 | $ 676,705,000
Gross unrealized gains | 32,503,000 | 13,567,000
Gross unrealized losses | (1,035,000) | (1,891,000)
Allowance for credit losses for investments | 0 |
Fair Value | 1,160,521,000 | 688,381,000
U.S. government agency securities |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized cost | 2,000,000 |
Gross unrealized gains | 0 |
Gross unrealized losses | (6,000) |
Allowance for credit losses for investments | 0 |
Fair Value | 1,994,000 |
Mortgage-backed securities - residential |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized cost | 723,333,000 | 474,144,000
Gross unrealized gains | 15,093,000 | 4,829,000
Gross unrealized losses | (320,000) | (1,661,000)
Allowance for credit losses for investments | 0 |
Fair Value | 738,106,000 | 477,312,000
Mortgage-backed securities - commercial |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized cost | 20,478,000 | 12,957,000
Gross unrealized gains | 1,376,000 | 407,000
Gross unrealized losses | 0 | 0
Allowance for credit losses for investments | 0 |
Fair Value | 21,854,000 | 13,364,000
Municipals, tax exempt |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized cost | 359,761,000 | 181,178,000
Gross unrealized gains | 15,813,000 | 8,287,000
Gross unrealized losses | (694,000) | (230,000)
Allowance for credit losses for investments | 0 |
Fair Value | 374,880,000 | 189,235,000
Treasury securities |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized cost | 21,482,000 | 7,426,000
Gross unrealized gains | 218,000 | 22,000
Gross unrealized losses | 0 | 0
Allowance for credit losses for investments | 0 |
Fair Value | 21,700,000 | 7,448,000
Corporate securities |  |
Debt Securities, Available-for-sale [Abstract] |  |
Amortized cost | 1,999,000 | 1,000,000
Gross unrealized gains | 3,000 | 22,000
Gross unrealized losses | (15,000) | 0
Allowance for credit losses for investments | 0 |
Fair Value | $ 1,987,000 | $ 1,022,000

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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