Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 18,924,711	$ 17,575,507
Available-for-sale Securities	19,619,084	17,708,156
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	30,211,787	28,322,977
Available-for-sale Securities, Gross Unrealized Gains	2,243,037	1,939,175
Available-for-sale Securities, Gross Unrealized Losses	361,199	619,247
Available-for-sale Securities	$ 32,093,625	$ 29,642,905
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	$ 100	$ 1,555
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	285,579	126,229
Available-for-sale Securities, Gross Unrealized Gains	3,440	7,837
Available-for-sale Securities, Gross Unrealized Losses	13,658	8,204
Available-for-sale Securities	$ 275,361	$ 125,862
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	$ 18,924,711	$ 17,575,507
Available-for-sale Securities, Gross Unrealized Gains	911,618	599,718
Available-for-sale Securities, Gross Unrealized Losses	217,245	467,069
Available-for-sale Securities	$ 19,619,084	$ 17,708,156
Percentage Of Total Available For Sale Securities	61.10%	59.70%
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	2,561,605	2,469,009
Available-for-sale Securities, Gross Unrealized Gains	1,085,982	1,110,282
Available-for-sale Securities, Gross Unrealized Losses	3,541	2,532
Available-for-sale Securities	$ 3,644,046	$ 3,576,759
Percentage Of Total Available For Sale Securities	11.40%	12.10%
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	1,258,039	1,277,998
Available-for-sale Securities, Gross Unrealized Gains	33,917	45,152
Available-for-sale Securities, Gross Unrealized Losses	13,380	11,673
Available-for-sale Securities	$ 1,278,576	$ 1,311,477
Percentage Of Total Available For Sale Securities	4.00%	4.40%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 375	$ 300
Asset-backed Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,443,822	1,219,000
Available-for-sale Securities, Gross Unrealized Gains	9,350	12,052
Available-for-sale Securities, Gross Unrealized Losses	23,828	18,376
Available-for-sale Securities	$ 1,429,344	$ 1,212,676
Percentage Of Total Available For Sale Securities	4.50%	4.10%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 275	$ 354
Commercial Mortgage Backed Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,342,440	1,456,848
Available-for-sale Securities, Gross Unrealized Gains	28,973	37,407
Available-for-sale Securities, Gross Unrealized Losses	7,759	11,168
Available-for-sale Securities	$ 1,363,654	$ 1,483,087
Percentage Of Total Available For Sale Securities	4.20%	5.00%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 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	1,518,702	1,423,791
Available-for-sale Securities, Gross Unrealized Gains	12,644	15,586
Available-for-sale Securities, Gross Unrealized Losses	63,044	57,718
Available-for-sale Securities	$ 1,468,302	$ 1,381,659
Percentage Of Total Available For Sale Securities	4.60%	4.70%
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	566,761	480,067
Available-for-sale Securities, Gross Unrealized Gains	37,499	40,014
Available-for-sale Securities, Gross Unrealized Losses	12,464	9,067
Available-for-sale Securities	$ 591,796	$ 511,014
Percentage Of Total Available For Sale Securities	1.80%	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	2,595,707	2,420,757
Available-for-sale Securities, Gross Unrealized Gains	123,054	78,964
Available-for-sale Securities, Gross Unrealized Losses	19,938	41,644
Available-for-sale Securities	$ 2,698,823	$ 2,458,077
Percentage Of Total Available For Sale Securities	8.40%	8.30%
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	55,812	85,645
Available-for-sale Securities, Gross Unrealized Gains	1,648	7,837
Available-for-sale Securities, Gross Unrealized Losses	6,337	5,962
Available-for-sale Securities	$ 51,123	$ 87,520
Percentage Of Total Available For Sale Securities	18.60%	69.50%
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	$ 229,767	$ 40,584
Available-for-sale Securities, Gross Unrealized Gains	1,792	0
Available-for-sale Securities, Gross Unrealized Losses	7,321	2,242
Available-for-sale Securities	$ 224,238	$ 38,342
Percentage Of Total Available For Sale Securities	81.40%	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
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669686-108580

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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
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-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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