Investments - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets measured at Fair Value
Available-for-sale fixed income	$ 2,175,869	$ 2,196,626
Equity securities	555,209	524,006
Municipal
Assets measured at Fair Value
Available-for-sale fixed income	544,579	532,396
Fair Value Measured on Recurring Basis
Assets measured at Fair Value
Available-for-sale fixed income	2,175,869	2,196,626
Equity securities	555,209	524,006
Other invested assets		6,068
Total assets at fair value	2,731,078	2,726,700
Fair Value Measured on Recurring Basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets measured at Fair Value
Equity securities	555,105	523,923
Other invested assets		6,068
Total assets at fair value	555,105	529,991
Fair Value Measured on Recurring Basis \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	2,153,417	2,178,828
Equity securities	104	83
Total assets at fair value	2,153,521	2,178,911
Fair Value Measured on Recurring Basis \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	22,452	17,798
Total assets at fair value	22,452	17,798
Fair Value Measured on Recurring Basis \| U.S. government
Assets measured at Fair Value
Available-for-sale fixed income	170,422	183,357
Fair Value Measured on Recurring Basis \| U.S. government \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	170,422	183,357
Fair Value Measured on Recurring Basis \| U.S. Agency
Assets measured at Fair Value
Available-for-sale fixed income	31,666	32,872
Fair Value Measured on Recurring Basis \| U.S. Agency \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	31,666	32,872
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency
Assets measured at Fair Value
Available-for-sale fixed income	8,555	10,965
Fair Value Measured on Recurring Basis \| Non-U.S. government & agency \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	8,555	10,965
Fair Value Measured on Recurring Basis \| Agency MBS
Assets measured at Fair Value
Available-for-sale fixed income	369,212	402,071
Fair Value Measured on Recurring Basis \| Agency MBS \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	369,212	402,071
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS
Assets measured at Fair Value
Available-for-sale fixed income	218,232	218,373
Fair Value Measured on Recurring Basis \| ABS/CMBS/MBS \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	218,232	218,373
Fair Value Measured on Recurring Basis \| Corporate Debt
Assets measured at Fair Value
Available-for-sale fixed income	833,203	816,592
Fair Value Measured on Recurring Basis \| Corporate Debt \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	810,751	798,794
Fair Value Measured on Recurring Basis \| Corporate Debt \| Significant Unobservable Inputs (Level 3)
Assets measured at Fair Value
Available-for-sale fixed income	22,452	17,798
Fair Value Measured on Recurring Basis \| Municipal
Assets measured at Fair Value
Available-for-sale fixed income	544,579	532,396
Fair Value Measured on Recurring Basis \| Municipal \| Significant Other Observable Inputs (Level 2)
Assets measured at Fair Value
Available-for-sale fixed income	$ 544,579	$ 532,396

Fair value of other invested assets.

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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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
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-Publisher FASB
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-Name Accounting Standards Codification
-Topic 320
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-Section 45
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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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