Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 24,216,876	$ 24,006,407
Estimated Fair Value	$ 25,077,338	$ 23,982,119
% of Total	100.00%	100.00%
Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 39,188,627	$ 38,882,168
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,780,039	1,858,668
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	230,223	748,490
Estimated Fair Value	$ 41,738,443	$ 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	24,216,876	24,006,407
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,024,557	530,804
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	164,095	555,092
Estimated Fair Value	$ 25,077,338	$ 23,982,119
% of Total	60.10%	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	2,870,018	2,768,466
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,408,501	1,126,227
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	655	2,308
Estimated Fair Value	$ 4,277,864	$ 3,892,385
% of Total	10.20%	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	1,969,694	1,872,236
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	34,746	22,267
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	10,254	25,282
Estimated Fair Value	$ 1,994,186	$ 1,869,221
% of Total	4.80%	4.70%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Asset-backed Securities [Member] \| Fixed maturity securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,243,449	2,171,254
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	14,286	10,779
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	15,749	32,829
Estimated Fair Value	$ 2,241,986	$ 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	1,404,852	1,428,115
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	30,009	9,153
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,115	18,234
Estimated Fair Value	$ 1,431,746	$ 1,419,034
% of Total	3.40%	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	1,578,975	2,233,537
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	9,826	10,204
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	21,238	57,867
Estimated Fair Value	$ 1,567,563	$ 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	738,690	721,290
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	56,668	39,914
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,641	9,010
Estimated Fair Value	$ 792,717	$ 752,194
% of Total	1.90%	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	4,166,073	3,680,863
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	201,446	109,320
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	12,476	47,868
Estimated Fair Value	$ 4,355,043	$ 3,742,315
% of Total	10.40%	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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-Publisher FASB
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-Name Accounting Standards Codification
-Topic 220
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-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669686-108580

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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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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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.

+ References

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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.

+ References

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