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Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Amortized Cost or Cost	$ 6,472,113
Carrying Amount	6,688,654
Short-term Investments [Member]
Amortized Cost or Cost	282,490
Fair Value	282,490
Carrying Amount	282,490
Other Investments [Member]
Amortized Cost or Cost	216,807
Carrying Amount	216,807
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	4,573
Fair Value	4,921
Carrying Amount	4,580
Held-to-maturity Securities [Member] | Public Utilities
Amortized Cost or Cost	3,608
Fair Value	3,967
Carrying Amount	3,673
Held-to-maturity Securities [Member] | All other corporate securities
Amortized Cost or Cost	12,588
Fair Value	13,087
Carrying Amount	12,547
Held-to-maturity Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	20,769
Fair Value	21,975
Carrying Amount	20,800
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member]
Amortized Cost or Cost	112,680
Fair Value	116,186
Carrying Amount	116,186
AFS Fixed Income Securities [Member] | Foreign Government
Amortized Cost or Cost	18,011
Fair Value	18,542
Carrying Amount	18,542
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	1,168,185
Fair Value	1,230,090
Carrying Amount	1,230,090
AFS Fixed Income Securities [Member] | Public Utilities
Amortized Cost or Cost	35,679
Fair Value	37,084
Carrying Amount	37,084
AFS Fixed Income Securities [Member] | All other corporate securities
Amortized Cost or Cost	1,831,202
Fair Value	1,910,393
Carrying Amount	1,910,393
AFS Fixed Income Securities [Member] | CLO and Other ABS
Amortized Cost or Cost	790,517
Fair Value	793,012
Carrying Amount	793,012
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Amortized Cost or Cost	514,709
Fair Value	538,344
Carrying Amount	538,344
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
Amortized Cost or Cost	1,409,003
Fair Value	1,451,969
Carrying Amount	1,451,969
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	5,879,986
Fair Value	6,095,620
Carrying Amount	6,095,620
Equity Securities [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	17,357
Fair Value	17,368
Carrying Amount	17,368
Equity Securities [Member] | Industrial, miscellaneous and all other
Amortized Cost or Cost	51,815
Fair Value	52,532
Carrying Amount	52,532
Equity Securities [Member] | Preferred Stock [Member]
Amortized Cost or Cost	2,889
Fair Value	3,037
Carrying Amount	3,037
Equity Securities [Member] | Equity Securities [Member]
Amortized Cost or Cost	72,061
Fair Value	72,937
Carrying Amount	$ 72,937

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=50485924&loc=d3e611379-123010

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- 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=50485924&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))
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010

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