| Pay vs Performance Disclosure - USD ($) | 12 Months Ended |
| Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 |
| Pay vs Performance Disclosure | | | | |
| Pay vs Performance Disclosure, Table | Pay Versus Performance In 2022, the SEC adopted rules requiring disclosure of information pertaining to both Principal Executive Officer (“PEO”) and other NEO compensation, including separate disclosure of the PEO’s total compensation and other NEOs’ average total compensation calculated in accordance with Item 402(c)(2)(x) of Regulation S-K. To determine Compensation Actually Paid (“CAP”) for the PEO and NEOs, we are required to make certain adjustments to total executive compensation reported in the Summary Compensation Table for equity awards that are calculated in accordance with US GAAP. The table below details CAP for our PEO, Michael Kehoe, the average of CAP to our other NEOs, our total shareholder return (“TSR”), the TSR of our peer group (the S&P 500 Property & Casualty Insurance Index), net income, and a company-selected measure. Our company-selected measure, the most important financial performance measure used to link compensation paid to all employees, including executive officers, to company performance for the most recently completed year, is actual underwriting profit. Actual underwriting profit is the performance metric used to determine the bonus pool calculation under the annual cash incentive program and from which the variable compensation is derived. Value of initial fixed $100 investment based on: Year Summary Compensation Table Total for PEO (1) Compensation Actually Paid to PEO (2) Average Summary Compensation Table Total for non-PEO NEOs (3) Average Total Compensation Actually Paid to non-PEO NEOs (2) Total share-holder return Peer group total share-holder return (4) Net income Actual Underwriting Profit 2023 5,223,077 6,126,808 2,223,567 2,556,774 $ 332.19 $ 168.05 308,093,000 269,797,812 2022 3,783,912 4,064,879 1,467,060 1,584,237 $ 258.97 $ 151.65 159,114,000 200,333,134 2021 3,028,861 3,427,664 1,274,034 1,417,695 $ 235.07 $ 127.58 152,659,000 152,209,751 2020 2,210,343 4,087,450 917,728 1,313,635 $ 197.28 $ 106.96 88,419,000 64,301,708 (1) Refers to total column reported in the Summary Compensation Table for Mr. Kehoe for each respective year. (2) Refer to CAP table below for a reconciliation of Summary Compensation Table compensation to CAP. (3) Average of total compensation reported in the Summary Compensation Table for non-PEO NEOs Brian Haney, Bryan Petrucelli, Mark Beachy and Diane Schnupp for each of 2020, 2021, 2022 and 2023. (4) Cumulative total returns to the S&P 500 Property & Casualty Insurance Index assuming an initial investment of $100 on December 31, 2019, the measurement period, and the reinvestment of dividends, if any. Components of Compensation Actually Paid The table below shows the reconciliation of Summary Compensation Table compensation to CAP for Mr. Kehoe and the average CAP for all other NEOs for the years presented in the CAP table. For outstanding and unvested awards as of end of covered fiscal year: Year Summary Compensation Table Total Less: Stock award value as reported in Summary Compensation Table Add: Fair value at year-end of any equity awards granted during fiscal year Add: Change in fair value of equity awards granted in prior years Add: Change in fair value of awards that vested during fiscal year Total Compensation Actually Paid PEO (5) : 2023 5,223,077 (1,799,975) 1,899,275 503,382 301,049 6,126,808 2022 3,783,912 (999,810) 1,245,097 184,125 (148,445) 4,064,879 2021 3,028,861 (749,883) 972,256 441,981 (265,551) 3,427,664 2020 2,210,343 (749,882) 995,647 1,349,925 281,416 4,087,450 Non-PEO 2023 2,223,567 (684,553) 722,317 187,713 107,730 2,556,774 NEOs (5) : 2022 1,467,060 (356,790) 444,323 63,328 (33,684) 1,584,237 2021 1,274,034 (299,944) 388,891 114,186 (59,472) 1,417,695 2020 917,728 (299,952) 372,592 269,069 54,198 1,313,635 (5) With respect to each 2023, 2022, 2021 and 2020, the reported PEO is Michael P. Kehoe and reported non-PEO NEOs are Brian Haney, Bryan Petrucelli, Mark Beachy and Diane Schnupp | | | |
| Company Selected Measure Name | Actual Underwriting Profit | | | |
| Named Executive Officers, Footnote | (3) Average of total compensation reported in the Summary Compensation Table for non-PEO NEOs Brian Haney, Bryan Petrucelli, Mark Beachy and Diane Schnupp for each of 2020, 2021, 2022 and 2023. | | | |
| Peer Group Issuers, Footnote | (4) Cumulative total returns to the S&P 500 Property & Casualty Insurance Index assuming an initial investment of $100 on December 31, 2019, the measurement period, and the reinvestment of dividends, if any. | | | |
| PEO Total Compensation Amount | $ 5,223,077 | $ 3,783,912 | $ 3,028,861 | $ 2,210,343 |
| PEO Actually Paid Compensation Amount | 6,126,808 | 4,064,879 | 3,427,664 | 4,087,450 |
| Non-PEO NEO Average Total Compensation Amount | 2,223,567 | 1,467,060 | 1,274,034 | 917,728 |
| Non-PEO NEO Average Compensation Actually Paid Amount | $ 2,556,774 | 1,584,237 | 1,417,695 | 1,313,635 |
| Compensation Actually Paid vs. Total Shareholder Return | Relationship of Compensation Actually Paid to Total Shareholder Return The following graph shows the relationship between executive compensation actually paid for both the CEO and average of other NEOs to our TSR and TSR for the S&P 500 Property & Casualty Insurance Index for the period from December 31, 2019 through December 31, 2023. The graph assumes an initial investment of $100 and the reinvestment of dividends, if any. | | | |
| Compensation Actually Paid vs. Company Selected Measure | The following graph depicts the relationship between the cash and equity portions of compensation of our PEO and average of cash and equity portions of compensation of our non-PEO NEOs and each of net income and actual underwriting profit. As a result of our compensation philosophy, we grant equity awards to certain employees, including executive officers, in order to align employee incentives with those of shareholders. In recent years, the price of our common stock has increased. As a result, CAP for both the CEO and average of NEOs is heavily influenced by the change in the price of our common stock. Our compensation philosophy, specifically the bonus component in the table above, is intended to align executive bonuses with the performance of our underwriting operations. Equity awards granted to employees are intended to incentivize management, including executive officers, to grow the business over the long-term. We do not view changes in the fair value of previously granted equity awards as a meaningful metric when evaluating executive compensation. | | | |
| Tabular List, Table | List of Most Important Performance Measures The following table lists the most important measures we used in determining executive compensation for the year ended December 31, 2023: Most Important Measures for Determining NEO Pay Actual underwriting profit (1) Combined ratio (2) Operating return on equity (3) (1) See CD&A for description of how actual underwriting profit is calculated. (2) Combined ratio is the sum of the loss ratio and the expense ratio. Loss ratio, expressed as a percentage, is the ratio of losses and loss adjustment expenses to sum of earned premiums and fee income, net of the effects of reinsurance. Expense ratio, expressed as a percentage, is the ratio of underwriting, acquisition and insurance expenses to the sum of net earned premiums and fee income. A combined ratio under 100% indicates an underwriting profit. A combined ratio over 100% indicates an underwriting loss. (3) Operating return on equity is a non-GAAP financial measure. We define operating return on equity as net operating earnings, a non-GAAP financial measure, expressed as a percentage of average beginning and ending stockholders’ equity during the period. See “Management’s Discussion and Analysis of Financial Condition and Results of Operations — Reconciliation of Non-GAAP Financial Measures” in our Annual Report on Form 10-K for the year ended December 31, 2023 for a reconciliation of net operating earnings and operating return on equity. | | | |
| Total Shareholder Return Amount | $ 332.19 | 258.97 | 235.07 | 197.28 |
| Peer Group Total Shareholder Return Amount | 168.05 | 151.65 | 127.58 | 106.96 |
| Net Income (Loss) | $ 308,093,000 | $ 159,114,000 | $ 152,659,000 | $ 88,419,000 |
| Company Selected Measure Amount | 269,797,812 | 200,333,134 | 152,209,751 | 64,301,708 |
| PEO Name | Michael Kehoe | | | |
| Measure:: 1 | | | | |
| Pay vs Performance Disclosure | | | | |
| Name | Actual underwriting profit (1) | | | |
| Measure:: 2 | | | | |
| Pay vs Performance Disclosure | | | | |
| Name | Combined ratio (2) | | | |
| Measure:: 3 | | | | |
| Pay vs Performance Disclosure | | | | |
| Name | Operating return on equity (3) | | | |
| PEO | Stock award value as reported in Summary Compensation Table [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | $ (1,799,975) | $ (999,810) | $ (749,883) | $ (749,882) |
| PEO | Fair value at year-end of any equity awards granted unvested [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 1,899,275 | 1,245,097 | 972,256 | 995,647 |
| PEO | Change in fair value of equity awards granted in prior years - unvested [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 503,382 | 184,125 | 441,981 | 1,349,925 |
| PEO | Change in fair value of awards that vested during fiscal year [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 301,049 | (148,445) | (265,551) | 281,416 |
| Non-PEO NEO | Stock award value as reported in Summary Compensation Table [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | (684,553) | (356,790) | (299,944) | (299,952) |
| Non-PEO NEO | Fair value at year-end of any equity awards granted unvested [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 722,317 | 444,323 | 388,891 | 372,592 |
| Non-PEO NEO | Change in fair value of equity awards granted in prior years - unvested [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | 187,713 | 63,328 | 114,186 | 269,069 |
| Non-PEO NEO | Change in fair value of awards that vested during fiscal year [Member] | | | | |
| Pay vs Performance Disclosure | | | | |
| Adjustment to Compensation, Amount | $ 107,730 | $ (33,684) | $ (59,472) | $ 54,198 |
