Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
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	$ 1,728,972	$ 10,578,982
Gross Unrealized Losses - Less than 12 months	53,258	513,764
Estimated Fair Value - 12 months or greater	2,169,909	822,840
Gross Unrealized Losses - 12 months or greater	136,912	105,483
Estimated Fair Value	3,898,881	11,401,822
Gross Unrealized Losses	190,170	619,247
Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	3,248	50,658
Gross Unrealized Losses - Less than 12 months	261	4,417
Estimated Fair Value - 12 months or greater	30,853	12,191
Gross Unrealized Losses - 12 months or greater	8,121	3,787
Estimated Fair Value	34,101	62,849
Gross Unrealized Losses	8,382	8,204
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	291,016	648,027
Gross Unrealized Losses - Less than 12 months	15,218	79,258
Estimated Fair Value - 12 months or greater	403,790	163,901
Gross Unrealized Losses - 12 months or greater	53,136	44,544
Estimated Fair Value	694,806	811,928
Gross Unrealized Losses	68,354	123,802
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	1,437,956	9,930,955
Gross Unrealized Losses - Less than 12 months	38,040	434,506
Estimated Fair Value - 12 months or greater	1,766,119	658,939
Gross Unrealized Losses - 12 months or greater	83,776	60,939
Estimated Fair Value	3,204,075	10,589,894
Gross Unrealized Losses	121,816	495,445
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	282,830	554,688
Gross Unrealized Losses - Less than 12 months	14,077	71,171
Estimated Fair Value - 12 months or greater	335,351	114,427
Gross Unrealized Losses - 12 months or greater	45,761	31,076
Estimated Fair Value	618,181	669,115
Gross Unrealized Losses	59,838	102,247
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	649,275	6,388,148
Gross Unrealized Losses - Less than 12 months	15,424	323,961
Estimated Fair Value - 12 months or greater	1,208,939	294,755
Gross Unrealized Losses - 12 months or greater	59,206	40,861
Estimated Fair Value	1,858,214	6,682,903
Gross Unrealized Losses	74,630	364,822
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		122,746
Gross Unrealized Losses - Less than 12 months		2,532
Estimated Fair Value - 12 months or greater		0
Gross Unrealized Losses - 12 months or greater		0
Estimated Fair Value		122,746
Gross Unrealized Losses		2,532
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	0	22,646
Gross Unrealized Losses - Less than 12 months	0	282
Estimated Fair Value - 12 months or greater	379	7,679
Gross Unrealized Losses - 12 months or greater	11	298
Estimated Fair Value	379	30,325
Gross Unrealized Losses	11	580
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	67,472	452,297
Gross Unrealized Losses - Less than 12 months	1,672	7,036
Estimated Fair Value - 12 months or greater	120,431	82,314
Gross Unrealized Losses - 12 months or greater	3,990	4,057
Estimated Fair Value	187,903	534,611
Gross Unrealized Losses	5,662	11,093
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	5,858	6,772
Gross Unrealized Losses - Less than 12 months	1,118	201
Estimated Fair Value - 12 months or greater	8,623	9,335
Gross Unrealized Losses - 12 months or greater	1,545	1,250
Estimated Fair Value	14,481	16,107
Gross Unrealized Losses	2,663	1,451
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	534,180	581,701
Gross Unrealized Losses - Less than 12 months	18,943	9,825
Estimated Fair Value - 12 months or greater	258,828	199,298
Gross Unrealized Losses - 12 months or greater	10,507	7,100
Estimated Fair Value	793,008	780,999
Gross Unrealized Losses	29,450	16,925
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		3,253
Gross Unrealized Losses - Less than 12 months		248
Estimated Fair Value - 12 months or greater		767
Gross Unrealized Losses - 12 months or greater		117
Estimated Fair Value		4,020
Gross Unrealized Losses		365
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	78,002	514,877
Gross Unrealized Losses - Less than 12 months	423	9,806
Estimated Fair Value - 12 months or greater	41,399	31,177
Gross Unrealized Losses - 12 months or greater	702	997
Estimated Fair Value	119,401	546,054
Gross Unrealized Losses	1,125	10,803
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		1,010,387
Gross Unrealized Losses - Less than 12 months		57,718
Estimated Fair Value - 12 months or greater		0
Gross Unrealized Losses - 12 months or greater		0
Estimated Fair Value		1,010,387
Gross Unrealized Losses		57,718
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	9,697	157,837
Gross Unrealized Losses - Less than 12 months	443	5,349
Estimated Fair Value - 12 months or greater	56,930	13,016
Gross Unrealized Losses - 12 months or greater	5,694	3,718
Estimated Fair Value	66,627	170,853
Gross Unrealized Losses	6,137	9,067
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	2,328	60,668
Gross Unrealized Losses - Less than 12 months	23	7,356
Estimated Fair Value - 12 months or greater	59,437	31,693
Gross Unrealized Losses - 12 months or greater	5,819	11,803
Estimated Fair Value	61,765	92,361
Gross Unrealized Losses	5,842	19,159
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	99,330	702,962
Gross Unrealized Losses - Less than 12 months	1,135	18,279
Estimated Fair Value - 12 months or greater	79,592	38,379
Gross Unrealized Losses - 12 months or greater	3,677	4,206
Estimated Fair Value	178,922	741,341
Gross Unrealized Losses	4,812	22,485
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	3,248	12,331
Gross Unrealized Losses - Less than 12 months	261	2,175
Estimated Fair Value - 12 months or greater	24,247	12,191
Gross Unrealized Losses - 12 months or greater	7,675	3,787
Estimated Fair Value	27,495	24,522
Gross Unrealized Losses	7,936	5,962
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	0	38,327
Gross Unrealized Losses - Less than 12 months	0	2,242
Estimated Fair Value - 12 months or greater	6,606	0
Gross Unrealized Losses - 12 months or greater	446	0
Estimated Fair Value	6,606	38,327
Gross Unrealized Losses	$ 446	$ 2,242