Investments (Investments in Fixed Maturity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 44,860,441	$ 38,882,168
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	4,723,069	1,858,668
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	102,243	748,490
Estimated Fair Value	$ 49,481,267	$ 39,992,346
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	$ 275	$ 275
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,154,301	24,006,407
Estimated Fair Value	$ 30,353,219	$ 23,982,119
% of Total	100.00%	100.00%
Corporate Debt Securities [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 28,154,301	$ 24,006,407
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,277,070	530,804
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	78,152	555,092
Estimated Fair Value	$ 30,353,219	$ 23,982,119
% of Total	61.30%	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	2,962,729	2,768,466
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,670,680	1,126,227
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	190	2,308
Estimated Fair Value	$ 4,633,219	$ 3,892,385
% of Total	9.40%	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	2,360,691	1,872,236
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	81,367	22,267
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,216	25,282
Estimated Fair Value	$ 2,439,842	$ 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	2,809,008	2,171,254
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	27,621	10,779
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13,994	32,829
Estimated Fair Value	$ 2,822,635	$ 2,149,204
% of Total	5.70%	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	1,698,334	1,428,115
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	84,345	9,153
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	461	18,234
Estimated Fair Value	$ 1,782,218	$ 1,419,034
% of Total	3.60%	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	1,570,540	2,233,537
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	98,193	10,204
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	228	57,867
Estimated Fair Value	$ 1,668,505	$ 2,185,874
% of Total	3.40%	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	1,078,019	721,290
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	113,684	39,914
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,600	9,010
Estimated Fair Value	$ 1,190,103	$ 752,194
% of Total	2.40%	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	4,226,819	3,680,863
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	370,109	109,320
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	5,402	47,868
Estimated Fair Value	$ 4,591,526	$ 3,742,315
% of Total	9.30%	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
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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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
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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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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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