Investments (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 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	$ 4,269,794	$ 4,095,470
Gross unrealized losses on investments in unrealized loss position	221,587	169,012
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation		113,584
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation		$ 55,428
Additional contractual commitments	$ 469,064
Sales Revenue, Net [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage threshold of unrealized losses not exceeded by any one single issuer	0.20%	0.20%
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	$ 60,407	$ 42,884
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate market value of investments in unrealized loss position	4,171,923	3,971,191	$ 4,171,923
Gross unrealized losses on investments in unrealized loss position	203,666	157,065	203,666
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	118,097	109,516	118,097
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	85,569	47,549	$ 85,569
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		58,891
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	43,353
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	$ 44,092	34,687
Sub-prime and Alt-A Loans [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation		$ 15