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Schedule I - Summay of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Amortized Cost or Cost	$ 5,300,143
Carrying Amount	5,364,947
Short-term Investments [Member]
Amortized Cost or Cost	221,701
Fair Value	221,701
Carrying Amount	221,701
Other Investments [Member]
Amortized Cost or Cost	102,397
Carrying Amount	102,397
Preferred Stock [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	16,226
Fair Value	16,145
Carrying Amount	16,145
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	77,466
Fair Value	79,916
Carrying Amount	77,783
Held-to-maturity Securities [Member] | Public Utilities
Amortized Cost or Cost	8,589
Fair Value	9,292
Carrying Amount	8,579
Held-to-maturity Securities [Member] | All other corporate securities
Amortized Cost or Cost	14,122
Fair Value	14,783
Carrying Amount	13,989
Held-to-maturity Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Amortized Cost or Cost	1,220
Fair Value	1,220
Carrying Amount	1,205
Held-to-maturity Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	101,397
Fair Value	105,211
Carrying Amount	101,556
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member]
Amortized Cost or Cost	75,139
Fair Value	77,333
Carrying Amount	77,333
AFS Fixed Income Securities [Member] | Foreign Government
Amortized Cost or Cost	26,559
Fair Value	26,865
Carrying Amount	26,865
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
Amortized Cost or Cost	1,366,287
Fair Value	1,379,593
Carrying Amount	1,379,593
AFS Fixed Income Securities [Member] | Public Utilities
Amortized Cost or Cost	108,664
Fair Value	110,000
Carrying Amount	110,000
AFS Fixed Income Securities [Member] | All other corporate securities
Amortized Cost or Cost	1,867,892
Fair Value	1,887,753
Carrying Amount	1,887,753
AFS Fixed Income Securities [Member] | CLO and Other ABS
Amortized Cost or Cost	527,876
Fair Value	528,960
Carrying Amount	528,960
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member]
Amortized Cost or Cost	256,356
Fair Value	256,842
Carrying Amount	256,842
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member]
Amortized Cost or Cost	4,753,759
Fair Value	4,792,540
Carrying Amount	4,792,540
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
Amortized Cost or Cost	524,986
Fair Value	525,194
Carrying Amount	525,194
Equity Securities [Member] | Common Stock [Member]
Amortized Cost or Cost	104,663
Fair Value	130,608
Carrying Amount	130,608
Equity Securities [Member] | Preferred Stock [Member]
Amortized Cost or Cost	16,226
Fair Value	16,145
Carrying Amount	16,145
Equity Securities [Member] | Equity Securities [Member]
Amortized Cost or Cost	120,889
Fair Value	146,753
Carrying Amount	146,753
Common Stock [Member] | Banks, trusts, and insurance companies
Amortized Cost or Cost	14,056
Fair Value	17,648
Carrying Amount	17,648
Common Stock [Member] | Industrial, miscellaneous and all other
Amortized Cost or Cost	90,607
Fair Value	112,960
Carrying Amount	$ 112,960

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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