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Investment Securities (Tables) | 6 Months Ended
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Jun. 30, 2019
Schedule of amortized cost and fair value of securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | June 30, 2019 | ​ | December 31, 2018
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(dollars in thousands) |  | Cost |  | Gains |  | Losses |  | Value |  | Cost |  | Gains |  | Losses |  | Value
U.S. Treasury securities | ​ | $ | 29,477 | ​ | $ | 79 | ​ | $ | — | ​ | $ | 29,556 | ​ | $ | 389,470 | ​ | $ | — | ​ | $ | — | ​ | $ | 389,470
Government agency debt securities | ​ | ​ | 24,583 | ​ | ​ | 130 | ​ | ​ | — | ​ | ​ | 24,713 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | —
Government-sponsored enterprises debt securities | ​ | ​ | 164,719 | ​ | ​ | 1 | ​ | ​ | (404) | ​ | ​ | 164,316 | ​ | ​ | 248,372 | ​ | ​ | — | ​ | ​ | (6,778) | ​ | ​ | 241,594
Government agency mortgage-backed securities | ​ | ​ | 363,295 | ​ | ​ | 2,043 | ​ | ​ | (3,117) | ​ | ​ | 362,221 | ​ | ​ | 426,710 | ​ | ​ | — | ​ | ​ | (15,174) | ​ | ​ | 411,536
Government-sponsored enterprises mortgage-backed securities | ​ | ​ | 152,506 | ​ | ​ | 852 | ​ | ​ | (1,675) | ​ | ​ | 151,683 | ​ | ​ | 156,056 | ​ | ​ | 85 | ​ | ​ | (5,294) | ​ | ​ | 150,847
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | 2,619,502 | ​ | ​ | 9,107 | ​ | ​ | (15,144) | ​ | ​ | 2,613,465 | ​ | ​ | 2,779,620 | ​ | ​ | — | ​ | ​ | (97,171) | ​ | ​ | 2,682,449
Government-sponsored enterprises | ​ | ​ | 1,045,515 | ​ | ​ | 8,549 | ​ | ​ | (4,542) | ​ | ​ | 1,049,522 | ​ | ​ | 620,337 | ​ | ​ | — | ​ | ​ | (17,745) | ​ | ​ | 602,592
Debt securities issued by states and political subdivisions | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 19,854 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 19,854
Total available-for-sale securities | ​ | $ | 4,399,597 | ​ | $ | 20,761 | ​ | $ | (24,882) | ​ | $ | 4,395,476 | ​ | $ | 4,640,419 | ​ | $ | 85 | ​ | $ | (142,162) | ​ | $ | 4,498,342
Schedule of amortized cost and fair value of debt securities by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | June 30, 2019
​ | ​ | Amortized | ​ | Fair
(dollars in thousands) |  | Cost |  | Value
Due in one year or less | ​ | $ | 29,477 | ​ | $ | 29,556
Due after one year through five years | ​ | ​ | 99,992 | ​ | ​ | 99,701
Due after five years through ten years | ​ | ​ | 64,727 | ​ | ​ | 64,615
Due after ten years | ​ | ​ | — | ​ | ​ | —
​ | ​ | ​ | 194,196 | ​ | ​ | 193,872
​ | ​ | ​ | ​ | ​ | ​ | ​
Government agency debt securities | ​ | ​ | 24,583 | ​ | ​ | 24,713
Government agency mortgage-backed securities | ​ | ​ | 363,295 | ​ | ​ | 362,221
Government-sponsored enterprises mortgage-backed securities | ​ | ​ | 152,506 | ​ | ​ | 151,683
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | 2,619,502 | ​ | ​ | 2,613,465
Government-sponsored enterprises | ​ | ​ | 1,045,515 | ​ | ​ | 1,049,522
Total collateralized mortgage obligations | ​ | ​ | 3,665,017 | ​ | ​ | 3,662,987
Total available-for-sale securities | ​ | $ | 4,399,597 | ​ | $ | 4,395,476
Schedule of gross unrealized losses and fair values of securities in a continuous loss position | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Time in Continuous Loss as of June 30, 2019
​ | ​ | Less Than 12 Months | ​ | 12 Months or More | ​ | Total
​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​
(dollars in thousands) |  | Losses |  | Fair Value |  | Losses |  | Fair Value |  | Losses |  | Fair Value
Government-sponsored enterprises debt securities | ​ | $ | — | ​ | $ | — | ​ | $ | (404) | ​ | $ | 76,290 | ​ | $ | (404) | ​ | $ | 76,290
Government agency mortgage-backed securities | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (3,117) | ​ | ​ | 154,670 | ​ | ​ | (3,117) | ​ | ​ | 154,670
Government-sponsored enterprises mortgage-backed securities | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (1,675) | ​ | ​ | 120,791 | ​ | ​ | (1,675) | ​ | ​ | 120,791
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | (181) | ​ | ​ | 76,844 | ​ | ​ | (14,963) | ​ | ​ | 1,407,580 | ​ | ​ | (15,144) | ​ | ​ | 1,484,424
Government-sponsored enterprises | ​ | ​ | (143) | ​ | ​ | 138,443 | ​ | ​ | (4,399) | ​ | ​ | 271,124 | ​ | ​ | (4,542) | ​ | ​ | 409,567
Total available-for-sale securities with unrealized losses | ​ | $ | (324) | ​ | $ | 215,287 | ​ | $ | (24,558) | ​ | $ | 2,030,455 | ​ | $ | (24,882) | ​ | $ | 2,245,742
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Time in Continuous Loss as of December 31, 2018
​ | ​ | Less Than 12 Months | ​ | 12 Months or More | ​ | Total
​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​
(dollars in thousands) |  | Losses |  | Fair Value |  | Losses |  | Fair Value |  | Losses |  | Fair Value
Government-sponsored enterprises debt securities | ​ | $ | — | ​ | $ | — | ​ | $ | (6,778) | ​ | $ | 157,939 | ​ | $ | (6,778) | ​ | $ | 157,939
Government agency mortgage-backed securities | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (15,174) | ​ | ​ | 373,891 | ​ | ​ | (15,174) | ​ | ​ | 373,891
Government-sponsored enterprises mortgage-backed securities | ​ | ​ | (1) | ​ | ​ | 172 | ​ | ​ | (5,293) | ​ | ​ | 125,869 | ​ | ​ | (5,294) | ​ | ​ | 126,041
Collateralized mortgage obligations: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Government agency | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (97,171) | ​ | ​ | 2,475,532 | ​ | ​ | (97,171) | ​ | ​ | 2,475,532
Government-sponsored enterprises | ​ | ​ | — | ​ | ​ | — | ​ | ​ | (17,745) | ​ | ​ | 486,175 | ​ | ​ | (17,745) | ​ | ​ | 486,175
Total available-for-sale securities with unrealized losses | ​ | $ | (1) | ​ | $ | 172 | ​ | $ | (142,161) | ​ | $ | 3,619,406 | ​ | $ | (142,162) | ​ | $ | 3,619,578

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- Definition Tabular disclosure of fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27290-111563 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922888-210455 + Details
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