Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Components of net investment income
Total investment income	$ 338,293	$ 321,866	$ 313,857
Investment expenses	(8,478)	(8,256)	(7,854)
Net investment income	329,815us-gaap_NetInvestmentIncome	313,610us-gaap_NetInvestmentIncome	306,003us-gaap_NetInvestmentIncome
Fixed maturities [Member]
Components of net investment income
Total investment income	317,756	304,024	297,042
Equity securities [Member]
Components of net investment income
Total investment income	4,849	3,698	2,814
Short-term and other investments [Member]
Components of net investment income
Total investment income	8,459	8,242	8,109
Other invested asset [Member]
Components of net investment income
Total investment income	$ 7,229	$ 5,902	$ 5,892

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.1-5)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Amount of expenses related to the generation of investment income.

+ 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)

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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