Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 17,926,454	$ 17,575,507
Estimated Fair Value	$ 18,512,048	$ 17,708,156
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 29,035,274	$ 28,322,977
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,490,906	1,939,175
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	377,466	619,247
Estimated Fair Value	$ 31,148,714	$ 29,642,905
% 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	124,022	126,229
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,918	7,837
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	9,064	8,204
Estimated Fair Value	$ 117,876	$ 125,862
% of Total	100.00%	100.00%
Corporate Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 17,926,454	$ 17,575,507
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	872,226	599,718
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	286,632	467,069
Estimated Fair Value	$ 18,512,048	$ 17,708,156
% of Total	59.50%	59.70%
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	2,611,654	2,469,009
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,269,576	1,110,282
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	113	2,532
Estimated Fair Value	$ 3,881,117	$ 3,576,759
% of Total	12.50%	12.10%
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	1,330,894	1,277,998
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	61,124	45,152
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	11,121	11,673
Estimated Fair Value	$ 1,380,897	$ 1,311,477
% of Total	4.40%	4.40%
Other-than-temporary impairments in AOCI	$ (300)	$ (300)
Asset-backed securities \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,319,616	1,219,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	10,462	12,052
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	40,697	18,376
Estimated Fair Value	$ 1,289,381	$ 1,212,676
% of Total	4.10%	4.10%
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	1,466,500	1,456,848
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	47,907	37,407
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,224	11,168
Estimated Fair Value	$ 1,508,183	$ 1,483,087
% of Total	4.80%	5.00%
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	1,469,170	1,423,791
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	40,859	15,586
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,949	57,718
Estimated Fair Value	$ 1,507,080	$ 1,381,659
% of Total	4.80%	4.70%
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	506,521	480,067
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	53,761	40,014
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,039	9,067
Estimated Fair Value	$ 552,243	$ 511,014
% 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	2,404,465	2,420,757
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	134,991	78,964
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	21,691	41,644
Estimated Fair Value	$ 2,517,765	$ 2,458,077
% of Total	8.10%	8.30%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Nonredeemable Preferred Stock [Member] \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	83,427	85,645
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,768	7,837
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,110	5,962
Estimated Fair Value	$ 78,085	$ 87,520
% of Total	66.20%	69.50%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 40,595	$ 40,584
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	150	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	954	2,242
Estimated Fair Value	$ 39,791	$ 38,342
% of Total	33.80%	30.50%

Percentage of available for sale securities

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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