Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 11,858,578	$ 11,697,394
Unrealized Losses	179,307	324,600
Estimated Fair Value	12,518,300	12,110,755
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,497,524	20,270,734
Unrealized Gains	1,834,642	1,518,301
Unrealized Losses	174,984	314,899
Estimated Fair Value	22,157,182	21,474,136
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	(4,168)	(4,168)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	276,144	409,472
Unrealized Gains	10,194	5,960
Unrealized Losses	4,323	9,701
Estimated Fair Value	282,015	405,731
% of Total	100.00%	100.00%
Corporate securities | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,858,578	11,697,394
Unrealized Gains	762,137	616,147
Unrealized Losses	102,415	202,786
Estimated Fair Value	12,518,300	12,110,755
% of Total	56.40%	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	2,655,865	2,728,111
Unrealized Gains	802,236	669,762
Unrealized Losses	7,937	16,848
Estimated Fair Value	3,450,164	3,381,025
% of Total	15.60%	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	969,944	970,434
Unrealized Gains	45,270	38,126
Unrealized Losses	13,343	18,917
Estimated Fair Value	1,001,871	989,643
% of Total	4.50%	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	933,130	891,751
Unrealized Gains	23,247	18,893
Unrealized Losses	11,798	15,812
Estimated Fair Value	944,579	894,832
% of Total	4.30%	4.20%
Other-than-temporary impairments in AOCI	(2,259)	(2,259)
Commercial Mortgage Backed Securities [Member] | Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,367,205	1,314,782
Unrealized Gains	95,154	91,651
Unrealized Losses	11,856	17,487
Estimated Fair Value	1,450,503	1,388,946
% of Total	6.50%	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	447,071	489,631
Unrealized Gains	18,158	16,468
Unrealized Losses	2,967	4,748
Estimated Fair Value	462,262	501,351
% of Total	2.10%	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	368,465	313,252
Unrealized Gains	31,367	21,907
Unrealized Losses	9,200	14,339
Estimated Fair Value	390,632	320,820
% of Total	1.80%	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	1,897,266	1,865,379
Unrealized Gains	57,073	45,347
Unrealized Losses	15,468	23,962
Estimated Fair Value	1,938,871	1,886,764
% of Total	8.80%	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	84,156	81,993
Unrealized Gains	7,273	5,342
Unrealized Losses	2,662	5,481
Estimated Fair Value	88,767	81,854
% of Total	31.50%	20.20%
Other equity securities | Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	191,988	327,479
Unrealized Gains	2,921	618
Unrealized Losses	1,661	4,220
Estimated Fair Value	$ 193,248	$ 323,877
% of Total	68.50%	79.80%