Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 16,000,887	$ 14,010,604
Unrealized Losses	393,841	134,919
Estimated Fair Value	$ 16,489,406	$ 14,885,583
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 26,305,332	$ 23,105,597
Unrealized Gains	2,147,882	2,508,673
Unrealized Losses	389,239	133,298
Estimated Fair Value	$ 28,063,975	$ 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	129,359	120,534
Unrealized Gains	2,911	7,947
Unrealized Losses	4,602	1,621
Estimated Fair Value	$ 127,668	$ 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	$ 16,000,887	$ 14,010,604
Unrealized Gains	730,409	965,523
Unrealized Losses	241,890	90,544
Estimated Fair Value	$ 16,489,406	$ 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	2,655,763	2,668,852
Unrealized Gains	1,140,904	1,196,420
Unrealized Losses	2,860	7
Estimated Fair Value	$ 3,793,807	$ 3,865,265
% of Total	13.50%	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	1,154,732	991,867
Unrealized Gains	47,241	52,640
Unrealized Losses	11,094	6,611
Estimated Fair Value	$ 1,190,879	$ 1,037,896
% of Total	4.20%	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	1,051,093	1,059,660
Unrealized Gains	19,915	20,301
Unrealized Losses	8,384	10,375
Estimated Fair Value	$ 1,062,624	$ 1,069,586
% of Total	3.80%	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	1,471,419	1,453,657
Unrealized Gains	63,699	87,593
Unrealized Losses	9,649	8,659
Estimated Fair Value	$ 1,525,469	$ 1,532,591
% of Total	5.40%	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	1,332,518	501,352
Unrealized Gains	18,149	25,014
Unrealized Losses	86,669	515
Estimated Fair Value	$ 1,263,998	$ 525,851
% of Total	4.50%	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	478,262	378,457
Unrealized Gains	38,891	51,117
Unrealized Losses	10,981	3,498
Estimated Fair Value	$ 506,172	$ 426,076
% of Total	1.80%	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	2,160,658	2,041,148
Unrealized Gains	88,674	110,065
Unrealized Losses	17,712	13,089
Estimated Fair Value	$ 2,231,620	$ 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	91,717	93,540
Unrealized Gains	2,477	7,350
Unrealized Losses	3,620	1,527
Estimated Fair Value	$ 90,574	$ 99,363
% of Total	70.90%	78.30%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 37,642	$ 26,994
Unrealized Gains	434	597
Unrealized Losses	982	94
Estimated Fair Value	$ 37,094	$ 27,497
% of Total	29.10%	21.70%

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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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