Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 19,281,539	$ 18,924,711
Estimated Fair Value	$ 20,161,453	$ 19,619,084
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 30,540,567	$ 30,211,787
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,425,426	2,243,037
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	271,200	361,199
Estimated Fair Value	$ 32,694,793	$ 32,093,625
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	$ 275	$ 100
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	117,129	285,579
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	892	3,440
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,405	13,658
Estimated Fair Value	$ 111,616	$ 275,361
% of Total	100.00%	100.00%
Corporate Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 19,281,539	$ 18,924,711
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,037,337	911,618
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	157,423	217,245
Estimated Fair Value	$ 20,161,453	$ 19,619,084
% of Total	61.80%	61.10%
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	2,601,403	2,561,605
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,122,418	1,085,982
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,457	3,541
Estimated Fair Value	$ 3,720,364	$ 3,644,046
% of Total	11.40%	11.40%
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	1,281,171	1,258,039
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	34,382	33,917
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	11,676	13,380
Estimated Fair Value	$ 1,303,877	$ 1,278,576
% of Total	4.00%	4.00%
Other-than-temporary impairments in AOCI	$ 0	$ 375
Asset-backed securities \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,379,251	1,443,822
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	14,480	9,350
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13,141	23,828
Estimated Fair Value	$ 1,380,590	$ 1,429,344
% of Total	4.20%	4.50%
Other-than-temporary impairments in AOCI	$ 275	$ 275
Commercial Mortgage Backed Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,272,020	1,342,440
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	24,353	28,973
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	7,035	7,759
Estimated Fair Value	$ 1,289,338	$ 1,363,654
% of Total	3.90%	4.20%
Other-than-temporary impairments in AOCI	$ 0	$ 0
US Treasury and Government [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,522,651	1,518,702
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	15,135	12,644
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	55,334	63,044
Estimated Fair Value	$ 1,482,452	$ 1,468,302
% of Total	4.50%	4.60%
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	566,553	566,761
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	40,471	37,499
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	11,567	12,464
Estimated Fair Value	$ 595,457	$ 591,796
% of Total	1.80%	1.80%
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	2,635,979	2,595,707
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	136,850	123,054
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	11,567	19,938
Estimated Fair Value	$ 2,761,262	$ 2,698,823
% of Total	8.40%	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	33,656	55,812
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	360	1,648
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,801	6,337
Estimated Fair Value	$ 31,215	$ 51,123
% of Total	28.00%	18.60%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 83,473	$ 229,767
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	532	1,792
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,604	7,321
Estimated Fair Value	$ 80,401	$ 224,238
% of Total	72.00%	81.40%

Percentage of available for sale securities

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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
-SubTopic 10
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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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