Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 15,801,362	$ 14,010,604
Unrealized Losses	452,058	134,919
Estimated Fair Value	$ 16,173,447	$ 14,885,583
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 25,844,795	$ 23,105,597
Unrealized Gains	2,009,994	2,508,673
Unrealized Losses	443,001	133,298
Estimated Fair Value	$ 27,411,788	$ 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	116,694	120,534
Unrealized Gains	2,737	7,947
Unrealized Losses	9,057	1,621
Estimated Fair Value	$ 110,374	$ 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	$ 15,801,362	$ 14,010,604
Unrealized Gains	702,697	965,523
Unrealized Losses	330,612	90,544
Estimated Fair Value	$ 16,173,447	$ 14,885,583
% of Total	59.10%	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	2,520,495	2,668,852
Unrealized Gains	1,023,160	1,196,420
Unrealized Losses	929	7
Estimated Fair Value	$ 3,542,726	$ 3,865,265
% of Total	12.90%	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	1,234,158	991,867
Unrealized Gains	55,711	52,640
Unrealized Losses	6,409	6,611
Estimated Fair Value	$ 1,283,460	$ 1,037,896
% of Total	4.70%	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	1,055,760	1,059,660
Unrealized Gains	16,971	20,301
Unrealized Losses	11,359	10,375
Estimated Fair Value	$ 1,061,372	$ 1,069,586
% of Total	3.90%	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	1,441,845	1,453,657
Unrealized Gains	58,555	87,593
Unrealized Losses	8,449	8,659
Estimated Fair Value	$ 1,491,951	$ 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	1,337,493	501,352
Unrealized Gains	20,688	25,014
Unrealized Losses	40,963	515
Estimated Fair Value	$ 1,317,218	$ 525,851
% of Total	4.80%	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	466,685	378,457
Unrealized Gains	42,564	51,117
Unrealized Losses	7,746	3,498
Estimated Fair Value	$ 501,503	$ 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	1,986,997	2,041,148
Unrealized Gains	89,648	110,065
Unrealized Losses	36,534	13,089
Estimated Fair Value	$ 2,040,111	$ 2,138,124
% of Total	7.40%	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	89,726	93,540
Unrealized Gains	2,737	7,350
Unrealized Losses	7,754	1,527
Estimated Fair Value	$ 84,709	$ 99,363
% of Total	76.70%	78.30%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 26,968	$ 26,994
Unrealized Gains	0	597
Unrealized Losses	1,303	94
Estimated Fair Value	$ 25,665	$ 27,497
% of Total	23.30%	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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