Investments (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Investment [Line Items]
Fixed maturity securities - available for sale, at market value	$ 5,326,118	$ 5,107,028
Percentage threshold that triggers a review of available for sale debt securities for declines in market value	80.00%
Aggregate market value of investments in unrealized loss position	503,685	845,092
Gross unrealized losses on investments in unrealized loss position	22,149	35,473
Percentage threshold of unrealized losses not exceeded by any one single issuer	0.03%	0.09%
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	2,647	13,710
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	19,500	21,758
Additional contractual commitments	71,741
Other invested assets, at fair value, shares	9,719,971
Percentage of shares representing the total outstanding shares	15.80%
Minimum [Member]
Investment [Line Items]
Limited partnership financial reporting lag time, months	1 month
Maximum [Member]
Investment [Line Items]
Limited partnership financial reporting lag time, months	3 months
Foreign Government Debt Securities [Member]
Investment [Line Items]
Fixed maturity securities - available for sale, at market value	812,688
Aggregate market value of investments in unrealized loss position	75,948	53,520
Gross unrealized losses on investments in unrealized loss position	4,176	2,602
European Sovereign Securities [Member]
Investment [Line Items]
Fixed maturity securities - available for sale, at market value	89,412
Percentage threshold maximum of securities held in sovereign debt	5.00%
France [Member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	48.60%
United Kingdom [Member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	15.10%
Sweden [member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	11.80%
Netherlands [Member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	7.30%
Austria [Member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	5.40%
Investment Grade [Member]
Investment [Line Items]
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	2,039	5,635
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	17,181	15,880
Sub-prime and Alt-A Loans [Member]
Investment [Line Items]
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	$ 25	$ 56

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