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, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 14,084,589	$ 14,010,604
Unrealized Losses	99,462	134,919
Estimated Fair Value	15,179,135us-gaap_AvailableForSaleSecurities	14,885,583us-gaap_AvailableForSaleSecurities
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,933,716	23,105,597
Unrealized Gains	2,965,520	2,508,673
Unrealized Losses	98,013	133,298
Estimated Fair Value	25,801,223	25,480,972
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	1,555	(1,555)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	130,038	120,534
Unrealized Gains	6,030	7,947
Unrealized Losses	1,449	1,621
Estimated Fair Value	134,619	126,860
% of Total	100.00%	100.00%
Corporate Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,084,589	14,010,604
Unrealized Gains	1,159,377	965,523
Unrealized Losses	64,831	90,544
Estimated Fair Value	15,179,135	14,885,583
% of Total	58.80%	58.40%
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	2,477,703	2,668,852
Unrealized Gains	1,416,036	1,196,420
Unrealized Losses	0	7
Estimated Fair Value	3,893,739	3,865,265
% of Total	15.10%	15.20%
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	1,072,415	991,867
Unrealized Gains	60,491	52,640
Unrealized Losses	5,232	6,611
Estimated Fair Value	1,127,674	1,037,896
% of Total	4.40%	4.10%
Other-than-temporary impairments in AOCI	(300)	(300)
Asset-backed securities \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,082,883	1,059,660
Unrealized Gains	23,322	20,301
Unrealized Losses	8,093	10,375
Estimated Fair Value	1,098,112	1,069,586
% of Total	4.30%	4.20%
Other-than-temporary impairments in AOCI	354	354
Commercial Mortgage Backed Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,448,372	1,453,657
Unrealized Gains	93,304	87,593
Unrealized Losses	7,402	8,659
Estimated Fair Value	1,534,274	1,532,591
% of Total	5.90%	6.00%
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	444,529	501,352
Unrealized Gains	28,879	25,014
Unrealized Losses	0	515
Estimated Fair Value	473,408	525,851
% of Total	1.80%	2.00%
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	378,403	378,457
Unrealized Gains	54,496	51,117
Unrealized Losses	3,128	3,498
Estimated Fair Value	429,771	426,076
% of Total	1.70%	1.70%
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	1,944,822	2,041,148
Unrealized Gains	129,615	110,065
Unrealized Losses	9,327	13,089
Estimated Fair Value	2,065,110	2,138,124
% of Total	8.00%	8.40%
Other-than-temporary impairments in AOCI	0	0
Nonredeemable Preferred Stock [Member] \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	90,643	93,540
Unrealized Gains	5,136	7,350
Unrealized Losses	1,405	1,527
Estimated Fair Value	94,374	99,363
% of Total	70.10%	78.30%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,395	26,994
Unrealized Gains	894	597
Unrealized Losses	44	94
Estimated Fair Value	$ 40,245	$ 27,497
% of Total	29.90%	21.70%

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- Definition

Accumulated other comprehensive loss related to portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized as available-for-sale that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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