Investments (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
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	$ 3,049,130		$ 6,372,183
Gross unrealized losses on investments in unrealized loss position	174,362		272,692
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	67,675		139,624
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	106,687		133,068
Gross unrealized depreciation	0		0
Additional contractual commitments	$ 447,875
Sales Revenue, Net [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage threshold of unrealized losses not exceeded by any one single issuer	5.00%	5.00%
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
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate market value of investments in unrealized loss position	$ 2,952,359		6,280,276
Gross unrealized losses on investments in unrealized loss position	161,388		255,960
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	67,675		139,624
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	$ 93,713		$ 116,336
Fixed Maturity Securities [Member] \| Sales Revenue, Net [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage threshold of unrealized losses not exceeded by any one single issuer	0.60%		0.70%
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	$ 16,509		$ 46,793
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	14,945		18,447
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	36,994		39,037
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	$ 67,220		$ 72,738

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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