Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	$ 7,603,263	$ 2,437,278
Gross Unrealized Losses - Less than 12 months	358,912	73,639
Estimated Fair Value - 12 months or greater	789,472	1,068,336
Gross Unrealized Losses - 12 months or greater	84,089	59,659
Estimated Fair Value	8,392,735	3,505,614
Gross Unrealized Losses	443,001	133,298
Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	64,476	11,619
Gross Unrealized Losses - Less than 12 months	6,784	235
Estimated Fair Value - 12 months or greater	6,411	22,645
Gross Unrealized Losses - 12 months or greater	2,273	1,386
Estimated Fair Value	70,887	34,264
Gross Unrealized Losses	9,057	1,621
Non-investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	799,305	469,408
Gross Unrealized Losses - Less than 12 months	78,061	33,311
Estimated Fair Value - 12 months or greater	162,505	51,539
Gross Unrealized Losses - 12 months or greater	35,848	10,805
Estimated Fair Value	961,810	520,947
Gross Unrealized Losses	113,909	44,116
Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	6,803,958	1,967,870
Gross Unrealized Losses - Less than 12 months	280,851	40,328
Estimated Fair Value - 12 months or greater	626,967	1,016,797
Gross Unrealized Losses - 12 months or greater	48,241	48,854
Estimated Fair Value	7,430,925	2,984,667
Gross Unrealized Losses	329,092	89,182
Corporate Debt Securities [Member] | Non-investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	659,513	415,886
Gross Unrealized Losses - Less than 12 months	60,972	29,316
Estimated Fair Value - 12 months or greater	112,210	32,567
Gross Unrealized Losses - 12 months or greater	22,565	3,404
Estimated Fair Value	771,723	448,453
Gross Unrealized Losses	83,537	32,720
Corporate Debt Securities [Member] | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	4,647,733	1,225,767
Gross Unrealized Losses - Less than 12 months	216,291	27,784
Estimated Fair Value - 12 months or greater	302,808	614,294
Gross Unrealized Losses - 12 months or greater	30,784	30,040
Estimated Fair Value	4,950,541	1,840,061
Gross Unrealized Losses	247,075	57,824
Canadian and Canadian provincial governments | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	81,615	0
Gross Unrealized Losses - Less than 12 months	929	0
Estimated Fair Value - 12 months or greater	0	1,235
Gross Unrealized Losses - 12 months or greater	0	7
Estimated Fair Value	81,615	1,235
Gross Unrealized Losses	929	7
Residential mortgage-backed securities | Non-investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	42,309	22,836
Gross Unrealized Losses - Less than 12 months	445	293
Estimated Fair Value - 12 months or greater	8,797	6,284
Gross Unrealized Losses - 12 months or greater	354	225
Estimated Fair Value	51,106	29,120
Gross Unrealized Losses	799	518
Residential mortgage-backed securities | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	231,621	78,864
Gross Unrealized Losses - Less than 12 months	2,363	846
Estimated Fair Value - 12 months or greater	66,339	135,414
Gross Unrealized Losses - 12 months or greater	3,247	5,247
Estimated Fair Value	297,960	214,278
Gross Unrealized Losses	5,610	6,093
Asset-backed securities | Non-investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	6,905	12,448
Gross Unrealized Losses - Less than 12 months	85	274
Estimated Fair Value - 12 months or greater	13,677	7,108
Gross Unrealized Losses - 12 months or greater	1,847	1,791
Estimated Fair Value	20,582	19,556
Gross Unrealized Losses	1,932	2,065
Asset-backed securities | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	289,210	332,785
Gross Unrealized Losses - Less than 12 months	3,234	4,021
Estimated Fair Value - 12 months or greater	182,478	109,411
Gross Unrealized Losses - 12 months or greater	6,193	4,289
Estimated Fair Value	471,688	442,196
Gross Unrealized Losses	9,427	8,310
Commercial mortgage-backed securities | Non-investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	3,238	3,288
Gross Unrealized Losses - Less than 12 months	262	249
Estimated Fair Value - 12 months or greater	7,280	5,580
Gross Unrealized Losses - 12 months or greater	3,566	5,385
Estimated Fair Value	10,518	8,868
Gross Unrealized Losses	3,828	5,634
Commercial mortgage-backed securities | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	222,799	78,632
Gross Unrealized Losses - Less than 12 months	3,499	564
Estimated Fair Value - 12 months or greater	24,236	28,375
Gross Unrealized Losses - 12 months or greater	1,122	2,461
Estimated Fair Value	247,035	107,007
Gross Unrealized Losses	4,621	3,025
US Government Agencies Debt Securities [Member] | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	909,226	81,317
Gross Unrealized Losses - Less than 12 months	40,963	89
Estimated Fair Value - 12 months or greater	0	32,959
Gross Unrealized Losses - 12 months or greater	0	426
Estimated Fair Value	909,226	114,276
Gross Unrealized Losses	40,963	515
US States and Political Subdivisions Debt Securities [Member] | Non-investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months		964
Gross Unrealized Losses - Less than 12 months		43
Estimated Fair Value - 12 months or greater		0
Gross Unrealized Losses - 12 months or greater		0
Estimated Fair Value		964
Gross Unrealized Losses		43
US States and Political Subdivisions Debt Securities [Member] | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	128,633	13,780
Gross Unrealized Losses - Less than 12 months	4,204	17
Estimated Fair Value - 12 months or greater	13,206	18,998
Gross Unrealized Losses - 12 months or greater	3,542	3,438
Estimated Fair Value	141,839	32,778
Gross Unrealized Losses	7,746	3,455
Foreign Government Debt Securities [Member] | Non-investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	87,340	13,986
Gross Unrealized Losses - Less than 12 months	16,297	3,136
Estimated Fair Value - 12 months or greater	20,541	0
Gross Unrealized Losses - 12 months or greater	7,516	0
Estimated Fair Value	107,881	13,986
Gross Unrealized Losses	23,813	3,136
Foreign Government Debt Securities [Member] | Investment grade securities | Fixed maturity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	293,121	156,725
Gross Unrealized Losses - Less than 12 months	9,368	7,007
Estimated Fair Value - 12 months or greater	37,900	76,111
Gross Unrealized Losses - 12 months or greater	3,353	2,946
Estimated Fair Value	331,021	232,836
Gross Unrealized Losses	12,721	9,953
Nonredeemable Preferred Stock [Member] | Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	38,857	11,619
Gross Unrealized Losses - Less than 12 months	5,481	235
Estimated Fair Value - 12 months or greater	6,411	19,100
Gross Unrealized Losses - 12 months or greater	2,273	1,292
Estimated Fair Value	45,268	30,719
Gross Unrealized Losses	7,754	1,527
Other equity securities | Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	25,619	0
Gross Unrealized Losses - Less than 12 months	1,303	0
Estimated Fair Value - 12 months or greater	0	3,545
Gross Unrealized Losses - 12 months or greater	0	94
Estimated Fair Value	25,619	3,545
Gross Unrealized Losses	$ 1,303	$ 94