Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 13,280,721	$ 11,697,394
Unrealized Losses	112,302	324,600
Estimated Fair Value	14,151,411	12,110,755
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,373,346	20,270,734
Unrealized Gains	2,216,795	1,518,301
Unrealized Losses	109,745	314,899
Estimated Fair Value	24,480,396	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	135,073	409,472
Unrealized Gains	8,228	5,960
Unrealized Losses	2,557	9,701
Estimated Fair Value	140,744	405,731
% of Total	100.00%	100.00%
Corporate securities | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,280,721	11,697,394
Unrealized Gains	932,793	616,147
Unrealized Losses	62,103	202,786
Estimated Fair Value	14,151,411	12,110,755
% of Total	57.80%	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	2,786,873	2,728,111
Unrealized Gains	964,710	669,762
Unrealized Losses	3,945	16,848
Estimated Fair Value	3,747,638	3,381,025
% of Total	15.30%	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	946,431	970,434
Unrealized Gains	49,676	38,126
Unrealized Losses	9,968	18,917
Estimated Fair Value	986,139	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	993,116	891,751
Unrealized Gains	24,314	18,893
Unrealized Losses	9,055	15,812
Estimated Fair Value	1,008,375	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	1,380,622	1,314,782
Unrealized Gains	103,032	91,651
Unrealized Losses	9,034	17,487
Estimated Fair Value	1,474,620	1,388,946
% of Total	6.00%	6.50%
Other-than-temporary impairments in AOCI	(1,609)	(1,609)
US government and agencies | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	429,215	489,631
Unrealized Gains	20,987	16,468
Unrealized Losses	1,696	4,748
Estimated Fair Value	448,506	501,351
% of Total	1.80%	2.30%
Other-than-temporary impairments in AOCI	0	0
State and political subdivisions | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	369,515	313,252
Unrealized Gains	42,553	21,907
Unrealized Losses	4,581	14,339
Estimated Fair Value	407,487	320,820
% of Total	1.70%	1.50%
Other-than-temporary impairments in AOCI	0	0
Other foreign government, supranational and foreign government-sponsored enterprises | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,186,853	1,865,379
Unrealized Gains	78,730	45,347
Unrealized Losses	9,363	23,962
Estimated Fair Value	2,256,220	1,886,764
% of Total	9.20%	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	81,240	81,993
Unrealized Gains	7,165	5,342
Unrealized Losses	1,567	5,481
Estimated Fair Value	86,838	81,854
% of Total	61.70%	20.20%
Other equity securities | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	53,833	327,479
Unrealized Gains	1,063	618
Unrealized Losses	990	4,220
Estimated Fair Value	$ 53,906	$ 323,877
% of Total	38.30%	79.80%