Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 4,124,496	$ 10,578,982
Gross Unrealized Losses - Less than 12 months	189,810	513,764
Estimated Fair Value - 12 months or greater	2,410,931	822,840
Gross Unrealized Losses - 12 months or greater	187,656	105,483
Estimated Fair Value	6,535,427	11,401,822
Gross Unrealized Losses	377,466	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	28,597	50,658
Gross Unrealized Losses - Less than 12 months	1,058	4,417
Estimated Fair Value - 12 months or greater	34,470	12,191
Gross Unrealized Losses - 12 months or greater	8,006	3,787
Estimated Fair Value	63,067	62,849
Gross Unrealized Losses	9,064	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	638,428	648,027
Gross Unrealized Losses - Less than 12 months	70,027	79,258
Estimated Fair Value - 12 months or greater	219,041	163,901
Gross Unrealized Losses - 12 months or greater	41,153	44,544
Estimated Fair Value	857,469	811,928
Gross Unrealized Losses	111,180	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	3,486,068	9,930,955
Gross Unrealized Losses - Less than 12 months	119,783	434,506
Estimated Fair Value - 12 months or greater	2,191,890	658,939
Gross Unrealized Losses - 12 months or greater	146,503	60,939
Estimated Fair Value	5,677,958	10,589,894
Gross Unrealized Losses	266,286	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	573,014	554,688
Gross Unrealized Losses - Less than 12 months	65,766	71,171
Estimated Fair Value - 12 months or greater	137,674	114,427
Gross Unrealized Losses - 12 months or greater	28,670	31,076
Estimated Fair Value	710,688	669,115
Gross Unrealized Losses	94,436	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	2,082,050	6,388,148
Gross Unrealized Losses - Less than 12 months	79,986	323,961
Estimated Fair Value - 12 months or greater	1,557,209	294,755
Gross Unrealized Losses - 12 months or greater	112,210	40,861
Estimated Fair Value	3,639,259	6,682,903
Gross Unrealized Losses	192,196	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	49,560	122,746
Gross Unrealized Losses - Less than 12 months	113	2,532
Estimated Fair Value - 12 months or greater	0	0
Gross Unrealized Losses - 12 months or greater	0	0
Estimated Fair Value	49,560	122,746
Gross Unrealized Losses	113	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	22,774	22,646
Gross Unrealized Losses - Less than 12 months	411	282
Estimated Fair Value - 12 months or greater	10,445	7,679
Gross Unrealized Losses - 12 months or greater	481	298
Estimated Fair Value	33,219	30,325
Gross Unrealized Losses	892	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	217,618	452,297
Gross Unrealized Losses - Less than 12 months	5,773	7,036
Estimated Fair Value - 12 months or greater	104,341	82,314
Gross Unrealized Losses - 12 months or greater	4,456	4,057
Estimated Fair Value	321,959	534,611
Gross Unrealized Losses	10,229	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	6,506	6,772
Gross Unrealized Losses - Less than 12 months	1,368	201
Estimated Fair Value - 12 months or greater	8,818	9,335
Gross Unrealized Losses - 12 months or greater	1,696	1,250
Estimated Fair Value	15,324	16,107
Gross Unrealized Losses	3,064	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	629,257	581,701
Gross Unrealized Losses - Less than 12 months	25,390	9,825
Estimated Fair Value - 12 months or greater	237,331	199,298
Gross Unrealized Losses - 12 months or greater	12,243	7,100
Estimated Fair Value	866,588	780,999
Gross Unrealized Losses	37,633	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	0	3,253
Gross Unrealized Losses - Less than 12 months	0	248
Estimated Fair Value - 12 months or greater	3,207	767
Gross Unrealized Losses - 12 months or greater	293	117
Estimated Fair Value	3,207	4,020
Gross Unrealized Losses	293	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	256,118	514,877
Gross Unrealized Losses - Less than 12 months	3,936	9,806
Estimated Fair Value - 12 months or greater	52,341	31,177
Gross Unrealized Losses - 12 months or greater	1,995	997
Estimated Fair Value	308,459	546,054
Gross Unrealized Losses	5,931	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	129,048	1,010,387
Gross Unrealized Losses - Less than 12 months	801	57,718
Estimated Fair Value - 12 months or greater	70,288	0
Gross Unrealized Losses - 12 months or greater	2,148	0
Estimated Fair Value	199,336	1,010,387
Gross Unrealized Losses	2,949	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	14,632	157,837
Gross Unrealized Losses - Less than 12 months	510	5,349
Estimated Fair Value - 12 months or greater	55,295	13,016
Gross Unrealized Losses - 12 months or greater	7,529	3,718
Estimated Fair Value	69,927	170,853
Gross Unrealized Losses	8,039	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	36,134	60,668
Gross Unrealized Losses - Less than 12 months	2,482	7,356
Estimated Fair Value - 12 months or greater	58,897	31,693
Gross Unrealized Losses - 12 months or greater	10,013	11,803
Estimated Fair Value	95,031	92,361
Gross Unrealized Losses	12,495	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	107,785	702,962
Gross Unrealized Losses - Less than 12 months	3,274	18,279
Estimated Fair Value - 12 months or greater	115,085	38,379
Gross Unrealized Losses - 12 months or greater	5,922	4,206
Estimated Fair Value	222,870	741,341
Gross Unrealized Losses	9,196	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	15,334	12,331
Gross Unrealized Losses - Less than 12 months	356	2,175
Estimated Fair Value - 12 months or greater	22,821	12,191
Gross Unrealized Losses - 12 months or greater	7,754	3,787
Estimated Fair Value	38,155	24,522
Gross Unrealized Losses	8,110	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	13,263	38,327
Gross Unrealized Losses - Less than 12 months	702	2,242
Estimated Fair Value - 12 months or greater	11,649	0
Gross Unrealized Losses - 12 months or greater	252	0
Estimated Fair Value	24,912	38,327
Gross Unrealized Losses	$ 954	$ 2,242