Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 21,252,180	$ 18,924,711
Estimated Fair Value	$ 22,341,661	$ 19,619,084
% of Total	100.00%	100.00%
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 33,738,334	$ 30,211,787
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2,779,535	2,243,037
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	172,443	361,199
Estimated Fair Value	$ 36,345,426	$ 32,093,625
% of Total	100.00%	100.00%
Other-than-temporary impairments in AOCI	$ 275	$ 100
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	109,958	285,579
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	957	3,440
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,638	13,658
Estimated Fair Value	$ 104,277	$ 275,361
% of Total	100.00%	100.00%
Corporate Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 21,252,180	$ 18,924,711
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,189,750	911,618
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	100,269	217,245
Estimated Fair Value	$ 22,341,661	$ 19,619,084
% of Total	61.50%	61.10%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Canadian and Canadian provincial governments \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,713,972	2,561,605
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,296,242	1,085,982
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	2,460	3,541
Estimated Fair Value	$ 4,007,754	$ 3,644,046
% of Total	11.00%	11.40%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Residential mortgage-backed securities \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,505,474	1,258,039
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	42,619	33,917
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,794	13,380
Estimated Fair Value	$ 1,539,299	$ 1,278,576
% of Total	4.20%	4.00%
Other-than-temporary impairments in AOCI	$ 0	$ 375
Asset-backed securities \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,630,499	1,443,822
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	17,266	9,350
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	5,924	23,828
Estimated Fair Value	$ 1,641,841	$ 1,429,344
% of Total	4.50%	4.50%
Other-than-temporary impairments in AOCI	$ 275	$ 275
Commercial Mortgage Backed Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,558,035	1,342,440
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	28,928	28,973
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	4,935	7,759
Estimated Fair Value	$ 1,582,028	$ 1,363,654
% of Total	4.40%	4.20%
Other-than-temporary impairments in AOCI	$ 0	$ 0
US Treasury and Government [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,738,419	1,518,702
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	15,193	12,644
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	32,048	63,044
Estimated Fair Value	$ 1,721,564	$ 1,468,302
% of Total	4.70%	4.60%
Other-than-temporary impairments in AOCI	$ 0	$ 0
US States and Political Subdivisions Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	599,622	566,761
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	47,564	37,499
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	8,216	12,464
Estimated Fair Value	$ 638,970	$ 591,796
% of Total	1.80%	1.80%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Foreign Government Debt Securities [Member] \| Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,740,133	2,595,707
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	141,973	123,054
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	9,797	19,938
Estimated Fair Value	$ 2,872,309	$ 2,698,823
% of Total	7.90%	8.40%
Other-than-temporary impairments in AOCI	$ 0	$ 0
Nonredeemable Preferred Stock [Member] \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,545	55,812
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	435	1,648
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,021	6,337
Estimated Fair Value	$ 31,959	$ 51,123
% of Total	30.60%	18.60%
Other equity securities \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 75,413	$ 229,767
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	522	1,792
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	3,617	7,321
Estimated Fair Value	$ 72,318	$ 224,238
% of Total	69.40%	81.40%

Percentage of available for sale securities

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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.

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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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