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Schedule I - Summay of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Amortized Cost or Cost	$ 5,560,500
Carrying Amount	5,685,179
Short-term Investments [Member]
Amortized Cost or Cost	165,555
Fair Value	165,555
Carrying Amount	165,555
Other Investments [Member]
Amortized Cost or Cost	132,268
Carrying Amount	132,268
Preferred Stock [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	14,115
Fair Value	14,948
Carrying Amount	14,948
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	25,154
Fair Value	26,261
Carrying Amount	25,238
Held-to-maturity Securities [Member] | Public Utilities
Amortized Cost or Cost	7,466
Fair Value	7,956
Carrying Amount	7,443
Held-to-maturity Securities [Member] | All other corporate securities
Amortized Cost or Cost	9,530
Fair Value	9,883
Carrying Amount	9,448
Held-to-maturity Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	42,150
Fair Value	44,100
Carrying Amount	42,129
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member]
Amortized Cost or Cost	49,326
Fair Value	49,740
Carrying Amount	49,740
AFS Fixed Income Securities [Member] | Foreign Government
Amortized Cost or Cost	18,040
Fair Value	18,555
Carrying Amount	18,555
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	1,539,307
Fair Value	1,582,970
Carrying Amount	1,582,970
AFS Fixed Income Securities [Member] | Public Utilities
Amortized Cost or Cost	50,071
Fair Value	51,035
Carrying Amount	51,035
AFS Fixed Income Securities [Member] | All other corporate securities
Amortized Cost or Cost	1,538,268
Fair Value	1,566,433
Carrying Amount	1,566,433
AFS Fixed Income Securities [Member] | CLO and Other ABS
Amortized Cost or Cost	789,152
Fair Value	795,458
Carrying Amount	795,458
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Amortized Cost or Cost	382,727
Fair Value	383,449
Carrying Amount	383,449
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
Amortized Cost or Cost	709,825
Fair Value	714,882
Carrying Amount	714,882
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	5,076,716
Fair Value	5,162,522
Carrying Amount	5,162,522
Equity Securities [Member] | Public Utilities
Amortized Cost or Cost	5,957
Fair Value	6,156
Carrying Amount	6,156
Equity Securities [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	34,301
Fair Value	40,510
Carrying Amount	40,510
Equity Securities [Member] | Industrial, miscellaneous and all other
Amortized Cost or Cost	89,438
Fair Value	121,091
Carrying Amount	121,091
Equity Securities [Member] | Common Stock [Member]
Amortized Cost or Cost	129,696
Fair Value	167,757
Carrying Amount	167,757
Equity Securities [Member] | Preferred Stock [Member]
Amortized Cost or Cost	14,115
Fair Value	14,948
Carrying Amount	14,948
Equity Securities [Member] | Equity Securities [Member]
Amortized Cost or Cost	143,811
Fair Value	182,705
Carrying Amount	$ 182,705

X

- Definition

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.

+ References

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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Period Type:	instant

X

- Definition

This item represents the original cost of the entity's investments including investments in equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts.

+ References

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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- Definition

This item represents the reported amount of the entity's investments measured at fair value. Fair value is the price that would be received to sell an investment in an orderly transaction between market participants at the balance sheet [measurement] date.

+ References

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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