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Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Amortized Cost or Cost	$ 5,948,735
Carrying Amount	5,960,651
Short-term Investments [Member]
Amortized Cost or Cost	323,864
Fair Value	323,864
Carrying Amount	323,864
Other Investments [Member]
Amortized Cost or Cost	178,938
Carrying Amount	178,938
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	17,431
Fair Value	17,969
Carrying Amount	17,470
Held-to-maturity Securities [Member] | Public Utilities
Amortized Cost or Cost	4,707
Fair Value	5,264
Carrying Amount	4,818
Held-to-maturity Securities [Member] | All other corporate securities
Amortized Cost or Cost	14,883
Fair Value	15,084
Carrying Amount	14,822
Held-to-maturity Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	37,021
Fair Value	38,317
Carrying Amount	37,110
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member]
Amortized Cost or Cost	120,092
Fair Value	121,310
Carrying Amount	121,310
AFS Fixed Income Securities [Member] | Foreign Government
Amortized Cost or Cost	23,202
Fair Value	23,131
Carrying Amount	23,131
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	1,121,615
Fair Value	1,138,469
Carrying Amount	1,138,469
AFS Fixed Income Securities [Member] | Public Utilities
Amortized Cost or Cost	28,464
Fair Value	28,393
Carrying Amount	28,393
AFS Fixed Income Securities [Member] | All other corporate securities
Amortized Cost or Cost	1,611,388
Fair Value	1,589,015
Carrying Amount	1,589,015
AFS Fixed Income Securities [Member] | CLO and Other ABS
Amortized Cost or Cost	720,193
Fair Value	717,362
Carrying Amount	717,362
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Amortized Cost or Cost	527,409
Fair Value	527,078
Carrying Amount	527,078
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
Amortized Cost or Cost	1,118,435
Fair Value	1,128,342
Carrying Amount	1,128,342
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	5,270,798
Fair Value	5,273,100
Carrying Amount	5,273,100
Equity Securities [Member] | Public Utilities
Amortized Cost or Cost	2,416
Fair Value	2,732
Carrying Amount	2,732
Equity Securities [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	7,448
Fair Value	8,331
Carrying Amount	8,331
Equity Securities [Member] | Industrial, miscellaneous and all other
Amortized Cost or Cost	125,361
Fair Value	133,664
Carrying Amount	133,664
Equity Securities [Member] | Preferred Stock [Member]
Amortized Cost or Cost	2,889
Fair Value	2,912
Carrying Amount	2,912
Equity Securities [Member] | Equity Securities [Member]
Amortized Cost or Cost	138,114
Fair Value	147,639
Carrying Amount	$ 147,639

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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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=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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