Segment Information (Income Before and After Federal Income Tax) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net investment income	$ 45,553	$ 41,430	$ 88,784	$ 78,849
Net realized and unrealized investment (losses) gains	(1,652)	1,734	(12,201)	689
Total investment income, before federal income tax	43,901	43,164	76,583	79,538
Tax on investment income	7,617	11,734	12,843	21,336
Total investment income, after federal income tax	36,284	31,430	63,740	58,202
Standard Commercial Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	41,016	34,759	47,820	77,305
Underwriting gain (loss), after federal income tax	$ 32,403	$ 22,593	$ 37,778	$ 50,248
Combined ratio	91.40%	92.20%	94.90%	91.20%
Standard Personal Lines [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ 4,805	$ (5,768)	$ 3,299	$ (662)
Underwriting gain (loss), after federal income tax	$ 3,796	$ (3,749)	$ 2,606	$ (430)
Combined ratio	93.70%	108.00%	97.80%	100.50%
Excess And Surplus Operations [Member]
Segment Reporting Information [Line Items]
Underwriting gain (loss), before federal income tax	$ (7,789)	$ 1,319	$ (8,354)	$ 2,889
Underwriting gain (loss), after federal income tax	$ (6,154)	$ 857	$ (6,600)	$ 1,878
Combined ratio	114.70%	97.50%	107.90%	97.20%

Taxes incurred on investment income

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The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses, after-tax

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No definition available.

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The income earned from investments in securities and other investments including dividends, interest, and returns on other investments, including realized gains/losses.

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After-tax amount of net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses.

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No definition available.

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Net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses.

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No definition available.

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Ratio of non-life losses and loss expenses, non-life acquisition expenses and non-life general and administrative expenses, to non-life net premiums earned.

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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (c)-(e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c),9(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-16(Column G))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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