Investments (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2012	Dec. 31, 2011
Investment [Line Items]
Fixed maturity securities - available for sale, at market value	$ 5,228,179	$ 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	797,096	845,092
Gross unrealized losses on investments in unrealized loss position	24,354	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	5,021	13,710
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	19,331	21,758
Additional contractual commitments	89,651
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 Securities [Member]
Investment [Line Items]
Fixed maturity securities - available for sale, at market value	800,017
Aggregate market value of investments in unrealized loss position	70,896	53,520
Gross unrealized losses on investments in unrealized loss position	2,773	2,602
European Sovereign Securities [Member]
Investment [Line Items]
Fixed maturity securities - available for sale, at market value	85,358
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	48.10%
United Kingdom [Member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	15.70%
Sweden [member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	12.20%
Netherlands [Member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	7.20%
Austria [Member]
Investment [Line Items]
Percentage of securities held in sovereign debt of a specific country	5.60%
Investment Grade [Member]
Investment [Line Items]
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	2,909	5,635
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	15,971	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	$ 57	$ 56

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Amount of debt securities categorized neither as held-to-maturity nor trading, held at market value.

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Limited partnership financial reporting lag time, months.

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Other Invested Assets, At Fair Value, Percentage Of Shares Representing Amount Of Total Outstanding Shares

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Percentage of securities held in sovereign debt of a specific country.

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Percentage threshold that triggers a review of available for sale debt securities for declines in market value.

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