Investments (Investments in Fixed Maturity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 26,055,078	$ 24,006,407
Estimated Fair Value	$ 27,635,431	$ 23,982,119
% of Total	100.00%	100.00%
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 42,457,618	$ 38,882,168
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	3,828,068	1,858,668
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	96,381	748,490
Estimated Fair Value	$ 46,189,305	$ 39,992,346
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	$ 275	$ 275
Corporate Debt Securities [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	26,055,078	24,006,407
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,651,141	530,804
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	70,788	555,092
Estimated Fair Value	$ 27,635,431	$ 23,982,119
% of Total	59.80%	59.90%
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	2,960,943	2,768,466
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,613,153	1,126,227
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	133	2,308
Estimated Fair Value	$ 4,573,963	$ 3,892,385
% of Total	9.90%	9.70%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Residential mortgage-backed securities \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,214,963	1,872,236
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	64,629	22,267
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,077	25,282
Estimated Fair Value	$ 2,276,515	$ 1,869,221
% of Total	4.90%	4.70%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Asset-backed securities \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,482,012	2,171,254
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	25,714	10,779
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13,062	32,829
Estimated Fair Value	$ 2,494,664	$ 2,149,204
% of Total	5.40%	5.40%
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	1,687,024	1,428,115
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	60,775	9,153
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	514	18,234
Estimated Fair Value	$ 1,747,285	$ 1,419,034
% of Total	3.80%	3.50%
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	1,738,348	2,233,537
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	40,202	10,204
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	370	57,867
Estimated Fair Value	$ 1,778,180	$ 2,185,874
% of Total	3.80%	5.50%
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	1,221,461	721,290
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	83,621	39,914
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,508	9,010
Estimated Fair Value	$ 1,303,574	$ 752,194
% of Total	2.90%	1.90%
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	4,097,789	3,680,863
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	288,833	109,320
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,929	47,868
Estimated Fair Value	$ 4,379,693	$ 3,742,315
% of Total	9.50%	9.40%
Other-than-temporary impairments in AOCI	$ 0	$ 0

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
-Paragraph 9A
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-Name Accounting Standards Codification
-Topic 220
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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
-Name Accounting Standards Codification
-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

No definition available.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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