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Shareholders' Equity - Convertible non-voting common equivalent preferred shares (Details) - shares |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2016 | Mar. 31, 2018 | Jun. 30, 2017 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Class of Stock [Line Items] |  |  |  |  |  |  |
Shares issued | [1] |  |  |  | 2,646,164 | 2,835,994 | 2,757,506
Number of shares converted | [2] |  |  |  | 0 | 0 | 17,022,600
Series D Preferred Shares |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Number of share in conversion |  |  |  |  | 10 |  |
Number of underlying common shares |  | 38,288,460 |  |  |  |  |
Shares issued |  |  | 17,022,600 | 21,265,860 |  |  |
Number of shares converted |  |  | 567,420 | 708,862 |  |  |
Number of preferred shares outstanding |  |  |  |  | 0 |  |
United Guaranty Corporation | Series D Preferred Shares |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Number of shares issued |  | 1,276,282 |  |  |  |  |
[1] | Includes shares issued from the exercise of stock options and stock appreciation rights, the vesting of restricted share units and shares issued from the employee share purchase plan.
[2] | Such shares represent common shares that were issued upon conversion of the non-voting common equivalent preference shares issued in connection with the AIG acquisition.

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- Definition Aggregate number of common shares reserved for future issuance. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition The number of shares converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4313-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4304-108586 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4332-108586 + Details
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- Definition Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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