Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 13,413,102	$ 11,697,394
Unrealized Losses	120,852	324,600
Estimated Fair Value	14,190,456	12,110,755
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,452,941	20,270,734
Unrealized Gains	2,140,434	1,518,301
Unrealized Losses	117,924	314,899
Estimated Fair Value	24,475,451	21,474,136
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	1,555	(4,168)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	156,962	409,472
Unrealized Gains	7,917	5,960
Unrealized Losses	2,928	9,701
Estimated Fair Value	161,951	405,731
% of Total	100.00%	100.00%
Corporate Debt Securities [Member] | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,413,102	11,697,394
Unrealized Gains	849,695	616,147
Unrealized Losses	72,341	202,786
Estimated Fair Value	14,190,456	12,110,755
% of Total	58.00%	56.40%
Other-than-temporary impairments in AOCI	0	0
Canadian and Canadian provincial governments | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,726,064	2,728,111
Unrealized Gains	989,925	669,762
Unrealized Losses	1,651	16,848
Estimated Fair Value	3,714,338	3,381,025
% of Total	15.20%	15.70%
Other-than-temporary impairments in AOCI	0	0
Residential mortgage-backed securities | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	964,436	970,434
Unrealized Gains	46,453	38,126
Unrealized Losses	10,172	18,917
Estimated Fair Value	1,000,717	989,643
% of Total	4.10%	4.60%
Other-than-temporary impairments in AOCI	(300)	(300)
Asset-backed securities | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	993,028	891,751
Unrealized Gains	22,054	18,893
Unrealized Losses	9,034	15,812
Estimated Fair Value	1,006,048	894,832
% of Total	4.10%	4.20%
Other-than-temporary impairments in AOCI	354	(2,259)
Commercial Mortgage Backed Securities [Member] | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,404,648	1,314,782
Unrealized Gains	85,609	91,651
Unrealized Losses	8,435	17,487
Estimated Fair Value	1,481,822	1,388,946
% of Total	6.00%	6.50%
Other-than-temporary impairments in AOCI	(1,609)	(1,609)
US Treasury and Government [Member] | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	462,675	489,631
Unrealized Gains	19,063	16,468
Unrealized Losses	1,752	4,748
Estimated Fair Value	479,986	501,351
% of Total	1.90%	2.30%
Other-than-temporary impairments in AOCI	0	0
US States and Political Subdivisions Debt Securities [Member] | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	369,631	313,252
Unrealized Gains	45,211	21,907
Unrealized Losses	4,778	14,339
Estimated Fair Value	410,064	320,820
% of Total	1.70%	1.50%
Other-than-temporary impairments in AOCI	0	0
Foreign Government Debt Securities [Member] | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,119,357	1,865,379
Unrealized Gains	82,424	45,347
Unrealized Losses	9,761	23,962
Estimated Fair Value	2,192,020	1,886,764
% of Total	9.00%	8.80%
Other-than-temporary impairments in AOCI	0	0
Nonredeemable Preferred Stock [Member] | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	82,236	81,993
Unrealized Gains	7,051	5,342
Unrealized Losses	1,698	5,481
Estimated Fair Value	87,589	81,854
% of Total	54.10%	20.20%
Other equity securities | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	74,726	327,479
Unrealized Gains	866	618
Unrealized Losses	1,230	4,220
Estimated Fair Value	$ 74,362	$ 323,877
% of Total	45.90%	79.80%