SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	$ 5,200,777
Fair Value	5,155,919
Amount at which shown in the balance sheet	5,155,919
Fixed income
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	5,196,784
Fair Value	5,151,987
Amount at which shown in the balance sheet	5,151,987
Fixed income \| U.S. Treasury securities and obligations of U.S. government corporations and agencies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	167,655
Fair Value	167,176
Amount at which shown in the balance sheet	167,176
Fixed income \| Obligations of U.S. states and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,701,826
Fair Value	1,720,100
Amount at which shown in the balance sheet	1,720,100
Fixed income \| Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	212,584
Fair Value	208,381
Amount at which shown in the balance sheet	208,381
Fixed income \| ABS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	111,953
Fair Value	112,033
Amount at which shown in the balance sheet	112,033
Fixed income \| Collateralized loan obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	310,587
Fair Value	305,295
Amount at which shown in the balance sheet	305,295
Fixed income \| Mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	465,590
Fair Value	446,621
Amount at which shown in the balance sheet	446,621
Fixed income \| All other corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,226,589
Fair Value	2,192,381
Amount at which shown in the balance sheet	2,192,381
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	3,993
Fair Value	3,932
Amount at which shown in the balance sheet	3,932
Equity Securities \| Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	3,993
Fair Value	3,932
Amount at which shown in the balance sheet	$ 3,932

This element represents the carrying (reported) amount of the entity's investments as of the balance sheet date. This amount may represent the individual investment's or group of similar investments' original cost, fair value, or such other amount at which the investment is shown in the entity's balance sheet.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611379-123010

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