Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 22,249,964	$ 21,966,803
Estimated Fair Value	$ 22,503,507	$ 23,210,968
% of Total	100.00%	100.00%
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 35,233,970	$ 35,281,412
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,143,192	2,982,754
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	592,208	113,346
Estimated Fair Value	$ 36,784,954	$ 38,150,820
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	$ 275	$ 275
Equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		102,841
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		958
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		3,647
Estimated Fair Value		$ 100,152
% of Total		100.00%
Corporate Debt Securities [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,249,964	$ 21,966,803
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	663,087	1,299,594
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	409,544	55,429
Estimated Fair Value	$ 22,503,507	$ 23,210,968
% of Total	61.20%	60.90%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Canadian and Canadian provincial governments \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,789,699	2,843,273
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,277,020	1,378,510
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,876	1,707
Estimated Fair Value	$ 4,062,843	$ 4,220,076
% of Total	11.00%	11.10%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Residential mortgage-backed securities \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,837,316	1,695,126
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	18,500	36,632
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	34,602	11,878
Estimated Fair Value	$ 1,821,214	$ 1,719,880
% of Total	5.00%	4.50%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Asset-backed securities \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,711,099	1,634,758
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	11,596	18,798
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13,871	5,194
Estimated Fair Value	$ 1,708,824	$ 1,648,362
% of Total	4.60%	4.30%
Other-than-temporary impairments in AOCI	$ 275	$ 275
Commercial Mortgage Backed Securities [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,249,616	1,285,594
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	8,591	22,627
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	15,698	4,834
Estimated Fair Value	$ 1,242,509	$ 1,303,387
% of Total	3.40%	3.40%
Other-than-temporary impairments in AOCI	$ 0	$ 0
US Treasury and Government [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,583,622	1,953,436
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	8,193	12,089
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	66,665	21,933
Estimated Fair Value	$ 1,525,150	$ 1,943,592
% of Total	4.10%	5.10%
Other-than-temporary impairments in AOCI	$ 0	$ 0
US States and Political Subdivisions Debt Securities [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	703,047	647,727
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	43,318	59,997
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	9,321	4,296
Estimated Fair Value	$ 737,044	$ 703,428
% of Total	2.00%	1.80%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Debt Security, Government, Non-US [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,109,607	3,254,695
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	112,887	154,507
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	38,631	8,075
Estimated Fair Value	$ 3,183,863	$ 3,401,127
% of Total	8.70%	8.90%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Nonredeemable Preferred Stock [Member] \| Equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		41,553
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		479
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		2,226
Estimated Fair Value		$ 39,806
% of Total		39.70%
Other equity securities \| Equity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 61,288
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		479
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		1,421
Estimated Fair Value		$ 60,346
% of Total		60.30%

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 320
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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.

+ References

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