Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 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	$ 7,743,786	$ 2,437,278
Gross Unrealized Losses - Less than 12 months	346,050	73,639
Estimated Fair Value - 12 months or greater	654,932	1,068,336
Gross Unrealized Losses - 12 months or greater	43,189	59,659
Estimated Fair Value	8,398,718	3,505,614
Gross Unrealized Losses	389,239	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	66,257	11,619
Gross Unrealized Losses - Less than 12 months	3,160	235
Estimated Fair Value - 12 months or greater	6,225	22,645
Gross Unrealized Losses - 12 months or greater	1,442	1,386
Estimated Fair Value	72,482	34,264
Gross Unrealized Losses	4,602	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	659,860	469,408
Gross Unrealized Losses - Less than 12 months	27,055	33,311
Estimated Fair Value - 12 months or greater	94,256	51,539
Gross Unrealized Losses - 12 months or greater	11,500	10,805
Estimated Fair Value	754,116	520,947
Gross Unrealized Losses	38,555	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	7,083,926	1,967,870
Gross Unrealized Losses - Less than 12 months	318,995	40,328
Estimated Fair Value - 12 months or greater	560,676	1,016,797
Gross Unrealized Losses - 12 months or greater	31,689	48,854
Estimated Fair Value	7,644,602	2,984,667
Gross Unrealized Losses	350,684	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	553,615	415,886
Gross Unrealized Losses - Less than 12 months	23,667	29,316
Estimated Fair Value - 12 months or greater	58,910	32,567
Gross Unrealized Losses - 12 months or greater	5,891	3,404
Estimated Fair Value	612,525	448,453
Gross Unrealized Losses	29,558	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	4,719,168	1,225,767
Gross Unrealized Losses - Less than 12 months	194,881	27,784
Estimated Fair Value - 12 months or greater	299,379	614,294
Gross Unrealized Losses - 12 months or greater	17,451	30,040
Estimated Fair Value	5,018,547	1,840,061
Gross Unrealized Losses	212,332	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	117,996	0
Gross Unrealized Losses - Less than 12 months	2,860	0
Estimated Fair Value - 12 months or greater	0	1,235
Gross Unrealized Losses - 12 months or greater	0	7
Estimated Fair Value	117,996	1,235
Gross Unrealized Losses	2,860	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	31,443	22,836
Gross Unrealized Losses - Less than 12 months	268	293
Estimated Fair Value - 12 months or greater	11,779	6,284
Gross Unrealized Losses - 12 months or greater	395	225
Estimated Fair Value	43,222	29,120
Gross Unrealized Losses	663	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	309,772	78,864
Gross Unrealized Losses - Less than 12 months	6,659	846
Estimated Fair Value - 12 months or greater	64,855	135,414
Gross Unrealized Losses - 12 months or greater	3,772	5,247
Estimated Fair Value	374,627	214,278
Gross Unrealized Losses	10,431	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	5,305	12,448
Gross Unrealized Losses - Less than 12 months	84	274
Estimated Fair Value - 12 months or greater	12,102	7,108
Gross Unrealized Losses - 12 months or greater	1,695	1,791
Estimated Fair Value	17,407	19,556
Gross Unrealized Losses	1,779	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	294,220	332,785
Gross Unrealized Losses - Less than 12 months	3,101	4,021
Estimated Fair Value - 12 months or greater	126,602	109,411
Gross Unrealized Losses - 12 months or greater	3,504	4,289
Estimated Fair Value	420,822	442,196
Gross Unrealized Losses	6,605	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	6,523	3,288
Gross Unrealized Losses - Less than 12 months	534	249
Estimated Fair Value - 12 months or greater	7,544	5,580
Gross Unrealized Losses - 12 months or greater	3,347	5,385
Estimated Fair Value	14,067	8,868
Gross Unrealized Losses	3,881	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	220,262	78,632
Gross Unrealized Losses - Less than 12 months	4,376	564
Estimated Fair Value - 12 months or greater	22,831	28,375
Gross Unrealized Losses - 12 months or greater	1,392	2,461
Estimated Fair Value	243,093	107,007
Gross Unrealized Losses	5,768	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	862,953	81,317
Gross Unrealized Losses - Less than 12 months	86,669	89
Estimated Fair Value - 12 months or greater	0	32,959
Gross Unrealized Losses - 12 months or greater	0	426
Estimated Fair Value	862,953	114,276
Gross Unrealized Losses	86,669	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	157,210	13,780
Gross Unrealized Losses - Less than 12 months	7,399	17
Estimated Fair Value - 12 months or greater	13,180	18,998
Gross Unrealized Losses - 12 months or greater	3,582	3,438
Estimated Fair Value	170,390	32,778
Gross Unrealized Losses	10,981	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	62,974	13,986
Gross Unrealized Losses - Less than 12 months	2,502	3,136
Estimated Fair Value - 12 months or greater	3,921	0
Gross Unrealized Losses - 12 months or greater	172	0
Estimated Fair Value	66,895	13,986
Gross Unrealized Losses	2,674	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	402,345	156,725
Gross Unrealized Losses - Less than 12 months	13,050	7,007
Estimated Fair Value - 12 months or greater	33,829	76,111
Gross Unrealized Losses - 12 months or greater	1,988	2,946
Estimated Fair Value	436,174	232,836
Gross Unrealized Losses	15,038	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	39,466	11,619
Gross Unrealized Losses - Less than 12 months	2,178	235
Estimated Fair Value - 12 months or greater	6,225	19,100
Gross Unrealized Losses - 12 months or greater	1,442	1,292
Estimated Fair Value	45,691	30,719
Gross Unrealized Losses	3,620	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	26,791	0
Gross Unrealized Losses - Less than 12 months	982	0
Estimated Fair Value - 12 months or greater	0	3,545
Gross Unrealized Losses - 12 months or greater	0	94
Estimated Fair Value	26,791	3,545
Gross Unrealized Losses	$ 982	$ 94