Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,966,803	$ 18,924,711
Available-for-sale Securities	23,210,968	19,619,084
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	35,281,412	30,211,787
Available-for-sale Securities, Gross Unrealized Gains	2,982,754	2,243,037
Available-for-sale Securities, Gross Unrealized Losses	113,346	361,199
Available-for-sale Securities	$ 38,150,820	$ 32,093,625
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	$ 275	$ 100
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	102,841	285,579
Available-for-sale Securities, Gross Unrealized Gains	958	3,440
Available-for-sale Securities, Gross Unrealized Losses	3,647	13,658
Available-for-sale Securities	$ 100,152	$ 275,361
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	$ 21,966,803	$ 18,924,711
Available-for-sale Securities, Gross Unrealized Gains	1,299,594	911,618
Available-for-sale Securities, Gross Unrealized Losses	55,429	217,245
Available-for-sale Securities	$ 23,210,968	$ 19,619,084
Percentage Of Total Available For Sale Securities	60.90%	61.10%
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	2,843,273	2,561,605
Available-for-sale Securities, Gross Unrealized Gains	1,378,510	1,085,982
Available-for-sale Securities, Gross Unrealized Losses	1,707	3,541
Available-for-sale Securities	$ 4,220,076	$ 3,644,046
Percentage Of Total Available For Sale Securities	11.10%	11.40%
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	1,695,126	1,258,039
Available-for-sale Securities, Gross Unrealized Gains	36,632	33,917
Available-for-sale Securities, Gross Unrealized Losses	11,878	13,380
Available-for-sale Securities	$ 1,719,880	$ 1,278,576
Percentage Of Total Available For Sale Securities	4.50%	4.00%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 375
Asset-backed Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,634,758	1,443,822
Available-for-sale Securities, Gross Unrealized Gains	18,798	9,350
Available-for-sale Securities, Gross Unrealized Losses	5,194	23,828
Available-for-sale Securities	$ 1,648,362	$ 1,429,344
Percentage Of Total Available For Sale Securities	4.30%	4.50%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 275	$ 275
Commercial Mortgage Backed Securities [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,285,594	1,342,440
Available-for-sale Securities, Gross Unrealized Gains	22,627	28,973
Available-for-sale Securities, Gross Unrealized Losses	4,834	7,759
Available-for-sale Securities	$ 1,303,387	$ 1,363,654
Percentage Of Total Available For Sale Securities	3.40%	4.20%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
US Treasury and Government [Member] \| Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,953,436	1,518,702
Available-for-sale Securities, Gross Unrealized Gains	12,089	12,644
Available-for-sale Securities, Gross Unrealized Losses	21,933	63,044
Available-for-sale Securities	$ 1,943,592	$ 1,468,302
Percentage Of Total Available For Sale Securities	5.10%	4.60%
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	647,727	566,761
Available-for-sale Securities, Gross Unrealized Gains	59,997	37,499
Available-for-sale Securities, Gross Unrealized Losses	4,296	12,464
Available-for-sale Securities	$ 703,428	$ 591,796
Percentage Of Total Available For Sale Securities	1.80%	1.80%
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	3,254,695	2,595,707
Available-for-sale Securities, Gross Unrealized Gains	154,507	123,054
Available-for-sale Securities, Gross Unrealized Losses	8,075	19,938
Available-for-sale Securities	$ 3,401,127	$ 2,698,823
Percentage Of Total Available For Sale Securities	8.90%	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	41,553	55,812
Available-for-sale Securities, Gross Unrealized Gains	479	1,648
Available-for-sale Securities, Gross Unrealized Losses	2,226	6,337
Available-for-sale Securities	$ 39,806	$ 51,123
Percentage Of Total Available For Sale Securities	39.70%	18.60%
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	$ 61,288	$ 229,767
Available-for-sale Securities, Gross Unrealized Gains	479	1,792
Available-for-sale Securities, Gross Unrealized Losses	1,421	7,321
Available-for-sale Securities	$ 60,346	$ 224,238
Percentage Of Total Available For Sale Securities	60.30%	81.40%

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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

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-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.

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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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