Investments (Narrative) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
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	$ 1,098,403	$ 1,892,146
Gross unrealized losses on investments in unrealized loss position	62,217	90,458
Percentage threshold of unrealized losses not exceeded by any one single issuer	0.50%us-gaap_ConcentrationRiskPercentage1	0.30%us-gaap_ConcentrationRiskPercentage1
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	51,748	72,254
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	10,469	18,204
Additional contractual commitments	237,637
Other invested assets, at fair value, shares	9,719,971
Percentage of shares representing the total outstanding shares	18.00%
Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Limited partnership financial reporting lag time, months	1 month
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Limited partnership financial reporting lag time, months	3 months
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	9,180	16,680
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate market value of investments in unrealized loss position	1,098,403	1,892,146
Gross unrealized losses on investments in unrealized loss position	62,217	90,458
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	51,748	72,254
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	10,469	18,204
Energy Sector [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	36,644	53,772
Foreign Exchange Losses [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	$ 13,713	$ 7,298

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