Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 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	$ 1,853,612	$ 10,578,982
Gross Unrealized Losses - Less than 12 months	29,088	513,764
Estimated Fair Value - 12 months or greater	1,303,016	822,840
Gross Unrealized Losses - 12 months or greater	101,195	105,483
Estimated Fair Value	3,156,628	11,401,822
Gross Unrealized Losses	130,283	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	100,048	50,658
Gross Unrealized Losses - Less than 12 months	306	4,417
Estimated Fair Value - 12 months or greater	30,592	12,191
Gross Unrealized Losses - 12 months or greater	6,754	3,787
Estimated Fair Value	130,640	62,849
Gross Unrealized Losses	7,060	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	327,014	648,027
Gross Unrealized Losses - Less than 12 months	9,885	79,258
Estimated Fair Value - 12 months or greater	226,442	163,901
Gross Unrealized Losses - 12 months or greater	50,335	44,544
Estimated Fair Value	553,456	811,928
Gross Unrealized Losses	60,220	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	1,526,598	9,930,955
Gross Unrealized Losses - Less than 12 months	19,203	434,506
Estimated Fair Value - 12 months or greater	1,076,574	658,939
Gross Unrealized Losses - 12 months or greater	50,860	60,939
Estimated Fair Value	2,603,172	10,589,894
Gross Unrealized Losses	70,063	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	314,962	554,688
Gross Unrealized Losses - Less than 12 months	9,132	71,171
Estimated Fair Value - 12 months or greater	168,865	114,427
Gross Unrealized Losses - 12 months or greater	46,168	31,076
Estimated Fair Value	483,827	669,115
Gross Unrealized Losses	55,300	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	775,973	6,388,148
Gross Unrealized Losses - Less than 12 months	11,522	323,961
Estimated Fair Value - 12 months or greater	567,398	294,755
Gross Unrealized Losses - 12 months or greater	29,794	40,861
Estimated Fair Value	1,343,371	6,682,903
Gross Unrealized Losses	41,316	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	369	7,679
Gross Unrealized Losses - 12 months or greater	10	298
Estimated Fair Value	369	30,325
Gross Unrealized Losses	10	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	85,970	452,297
Gross Unrealized Losses - Less than 12 months	593	7,036
Estimated Fair Value - 12 months or greater	107,240	82,314
Gross Unrealized Losses - 12 months or greater	3,236	4,057
Estimated Fair Value	193,210	534,611
Gross Unrealized Losses	3,829	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	731	201
Estimated Fair Value - 12 months or greater	14,707	9,335
Gross Unrealized Losses - 12 months or greater	2,360	1,250
Estimated Fair Value	20,565	16,107
Gross Unrealized Losses	3,091	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	341,260	581,701
Gross Unrealized Losses - Less than 12 months	4,816	9,825
Estimated Fair Value - 12 months or greater	314,518	199,298
Gross Unrealized Losses - 12 months or greater	11,488	7,100
Estimated Fair Value	655,778	780,999
Gross Unrealized Losses	16,304	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	51,965	514,877
Gross Unrealized Losses - Less than 12 months	193	9,806
Estimated Fair Value - 12 months or greater	22,415	31,177
Gross Unrealized Losses - 12 months or greater	425	997
Estimated Fair Value	74,380	546,054
Gross Unrealized Losses	618	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	86,499	1,010,387
Gross Unrealized Losses - Less than 12 months	161	57,718
Estimated Fair Value - 12 months or greater	0	0
Gross Unrealized Losses - 12 months or greater	0	0
Estimated Fair Value	86,499	1,010,387
Gross Unrealized Losses	161	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	53,229	157,837
Gross Unrealized Losses - Less than 12 months	841	5,349
Estimated Fair Value - 12 months or greater	13,141	13,016
Gross Unrealized Losses - 12 months or greater	3,577	3,718
Estimated Fair Value	66,370	170,853
Gross Unrealized Losses	4,418	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	6,194	60,668
Gross Unrealized Losses - Less than 12 months	22	7,356
Estimated Fair Value - 12 months or greater	42,501	31,693
Gross Unrealized Losses - 12 months or greater	1,797	11,803
Estimated Fair Value	48,695	92,361
Gross Unrealized Losses	1,819	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	131,702	702,962
Gross Unrealized Losses - Less than 12 months	1,077	18,279
Estimated Fair Value - 12 months or greater	51,862	38,379
Gross Unrealized Losses - 12 months or greater	2,340	4,206
Estimated Fair Value	183,564	741,341
Gross Unrealized Losses	3,417	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	3,413	12,331
Gross Unrealized Losses - Less than 12 months	96	2,175
Estimated Fair Value - 12 months or greater	23,786	12,191
Gross Unrealized Losses - 12 months or greater	6,508	3,787
Estimated Fair Value	27,199	24,522
Gross Unrealized Losses	6,604	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	96,635	38,327
Gross Unrealized Losses - Less than 12 months	210	2,242
Estimated Fair Value - 12 months or greater	6,806	0
Gross Unrealized Losses - 12 months or greater	246	0
Estimated Fair Value	103,441	38,327
Gross Unrealized Losses	$ 456	$ 2,242