Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 17,575,507	$ 14,010,604
Available-for-sale Securities	17,708,156	14,885,583
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	28,322,977	23,105,597
Available-for-sale Securities, Gross Unrealized Gains	1,939,175	2,508,673
Available-for-sale Securities, Gross Unrealized Losses	619,247	133,298
Available-for-sale Securities	$ 29,642,905	$ 25,480,972
Percentage Of Total Available For Sale Securities	100.00%	100.00%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 1,555	$ 1,555
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	126,229	120,534
Available-for-sale Securities, Gross Unrealized Gains	7,837	7,947
Available-for-sale Securities, Gross Unrealized Losses	8,204	1,621
Available-for-sale Securities	$ 125,862	$ 126,860
Percentage Of Total Available For Sale Securities	100.00%	100.00%
Corporate Debt Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 17,575,507	$ 14,010,604
Available-for-sale Securities, Gross Unrealized Gains	599,718	965,523
Available-for-sale Securities, Gross Unrealized Losses	467,069	90,544
Available-for-sale Securities	$ 17,708,156	$ 14,885,583
Percentage Of Total Available For Sale Securities	59.70%	58.40%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Canadian Provincial Governments [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,469,009	2,668,852
Available-for-sale Securities, Gross Unrealized Gains	1,110,282	1,196,420
Available-for-sale Securities, Gross Unrealized Losses	2,532	7
Available-for-sale Securities	$ 3,576,759	$ 3,865,265
Percentage Of Total Available For Sale Securities	12.10%	15.20%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Residential Mortgage Backed Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,277,998	991,867
Available-for-sale Securities, Gross Unrealized Gains	45,152	52,640
Available-for-sale Securities, Gross Unrealized Losses	11,673	6,611
Available-for-sale Securities	$ 1,311,477	$ 1,037,896
Percentage Of Total Available For Sale Securities	4.40%	4.10%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 300	$ 300
Asset-backed Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,219,000	1,059,660
Available-for-sale Securities, Gross Unrealized Gains	12,052	20,301
Available-for-sale Securities, Gross Unrealized Losses	18,376	10,375
Available-for-sale Securities	$ 1,212,676	$ 1,069,586
Percentage Of Total Available For Sale Securities	4.10%	4.20%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 354	$ 354
Commercial Mortgage Backed Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,456,848	1,453,657
Available-for-sale Securities, Gross Unrealized Gains	37,407	87,593
Available-for-sale Securities, Gross Unrealized Losses	11,168	8,659
Available-for-sale Securities	$ 1,483,087	$ 1,532,591
Percentage Of Total Available For Sale Securities	5.00%	6.00%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 1,609	$ 1,609
US Treasury and Government [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,423,791	501,352
Available-for-sale Securities, Gross Unrealized Gains	15,586	25,014
Available-for-sale Securities, Gross Unrealized Losses	57,718	515
Available-for-sale Securities	$ 1,381,659	$ 525,851
Percentage Of Total Available For Sale Securities	4.70%	2.00%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
US States and Political Subdivisions Debt Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	480,067	378,457
Available-for-sale Securities, Gross Unrealized Gains	40,014	51,117
Available-for-sale Securities, Gross Unrealized Losses	9,067	3,498
Available-for-sale Securities	$ 511,014	$ 426,076
Percentage Of Total Available For Sale Securities	1.70%	1.70%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Foreign Government Debt Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,420,757	2,041,148
Available-for-sale Securities, Gross Unrealized Gains	78,964	110,065
Available-for-sale Securities, Gross Unrealized Losses	41,644	13,089
Available-for-sale Securities	$ 2,458,077	$ 2,138,124
Percentage Of Total Available For Sale Securities	8.30%	8.40%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Nonredeemable Preferred Stock [Member] \| Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	85,645	93,540
Available-for-sale Securities, Gross Unrealized Gains	7,837	7,350
Available-for-sale Securities, Gross Unrealized Losses	5,962	1,527
Available-for-sale Securities	$ 87,520	$ 99,363
Percentage Of Total Available For Sale Securities	69.50%	78.30%
Equity Securities Other Than Non-redeemable Preferred Stock [Member] \| Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 40,584	$ 26,994
Available-for-sale Securities, Gross Unrealized Gains	0	597
Available-for-sale Securities, Gross Unrealized Losses	2,242	94
Available-for-sale Securities	$ 38,342	$ 27,497
Percentage Of Total Available For Sale Securities	30.50%	21.70%

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
-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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-Section 45
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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
-Section 25
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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