Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 18,762,648	$ 17,575,507
Estimated Fair Value	$ 20,164,848	$ 17,708,156
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 30,097,896	$ 28,322,977
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	3,568,806	1,939,175
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	130,283	619,247
Estimated Fair Value	$ 33,536,419	$ 29,642,905
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	$ (1,634)	$ (1,555)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	417,783	126,229
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	9,525	7,837
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	7,060	8,204
Estimated Fair Value	$ 420,248	$ 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	$ 18,762,648	$ 17,575,507
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,498,816	599,718
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	96,616	467,069
Estimated Fair Value	$ 20,164,848	$ 17,708,156
% of Total	60.20%	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	2,598,625	2,469,009
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,541,754	1,110,282
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	2,532
Estimated Fair Value	$ 4,140,379	$ 3,576,759
% of Total	12.30%	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	1,247,677	1,277,998
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	67,854	45,152
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,839	11,673
Estimated Fair Value	$ 1,311,692	$ 1,311,477
% of Total	3.90%	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	1,388,263	1,219,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	13,706	12,052
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	19,395	18,376
Estimated Fair Value	$ 1,382,574	$ 1,212,676
% of Total	4.10%	4.10%
Other-than-temporary impairments in AOCI	$ (275)	$ (354)
Commercial Mortgage Backed Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,402,249	1,456,848
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	73,402	37,407
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	618	11,168
Estimated Fair Value	$ 1,475,033	$ 1,483,087
% of Total	4.40%	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	1,479,295	1,423,791
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	80,402	15,586
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	161	57,718
Estimated Fair Value	$ 1,559,536	$ 1,381,659
% of Total	4.70%	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	549,669	480,067
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	72,598	40,014
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	4,418	9,067
Estimated Fair Value	$ 617,849	$ 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	2,669,470	2,420,757
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	220,274	78,964
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	5,236	41,644
Estimated Fair Value	$ 2,884,508	$ 2,458,077
% of Total	8.60%	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	56,944	85,645
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,512	7,837
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,604	5,962
Estimated Fair Value	$ 52,852	$ 87,520
% of Total	12.60%	69.50%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 360,839	$ 40,584
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	7,013	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	456	2,242
Estimated Fair Value	$ 367,396	$ 38,342
% of Total	87.40%	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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