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Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Amortized Cost or Cost	$ 7,114,400
Carrying Amount	7,505,599
Short-term Investments [Member]
Amortized Cost or Cost	409,865
Carrying Amount	409,852
Other Investments [Member]
Amortized Cost or Cost	266,322
Carrying Amount	266,322
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	4,505
Fair Value	4,795
Carrying Amount	4,507
Held-to-maturity Securities [Member] | Public Utilities
Amortized Cost or Cost	2,436
Fair Value	2,579
Carrying Amount	2,464
Held-to-maturity Securities [Member] | All other corporate securities
Amortized Cost or Cost	9,898
Fair Value	10,627
Carrying Amount	9,853
Held-to-maturity Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	16,839
Fair Value	18,001
Carrying Amount	16,824
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member]
Amortized Cost or Cost	110,038
Fair Value	116,140
Carrying Amount	116,140
AFS Fixed Income Securities [Member] | Foreign Government
Amortized Cost or Cost	16,801
Fair Value	18,366
Carrying Amount	18,366
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	1,159,588
Fair Value	1,247,137
Carrying Amount	1,247,137
AFS Fixed Income Securities [Member] | Public Utilities
Amortized Cost or Cost	68,269
Fair Value	73,821
Carrying Amount	73,821
AFS Fixed Income Securities [Member] | All other corporate securities
Amortized Cost or Cost	2,083,934
Fair Value	2,254,231
Carrying Amount	2,254,231
AFS Fixed Income Securities [Member] | CLO and Other ABS
Amortized Cost or Cost	1,014,820
Fair Value	1,026,551
Carrying Amount	1,026,551
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Amortized Cost or Cost	999,485
Fair Value	1,051,788
Carrying Amount	1,051,788
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
Amortized Cost or Cost	620,582
Fair Value	667,894
Carrying Amount	667,894
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	6,073,517
Fair Value	6,455,928
Carrying Amount	6,455,928
Equity Securities [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	18,366
Fair Value	17,474
Carrying Amount	17,474
Equity Securities [Member] | Industrial, miscellaneous and all other
Amortized Cost or Cost	281,619
Fair Value	291,158
Carrying Amount	291,158
Equity Securities [Member] | Preferred Stock [Member]
Amortized Cost or Cost	1,566
Fair Value	1,735
Carrying Amount	1,735
Equity Securities [Member] | Equity Securities [Member]
Amortized Cost or Cost	301,551
Fair Value	310,367
Carrying Amount	310,367
Commercial Mortgage Loans [Member]
Amortized Cost or Cost	46,306
Carrying Amount	$ 46,306

X

- Definition

Amount of investment held by insurance company, excluding investment in related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column D))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611379-123010

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X

- Definition

Cost of investment held by insurance company, excluding investment in related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column B))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611379-123010

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column C))
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