Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
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	$ 5,317,458	$ 8,327,843
Gross Unrealized Losses - Less than 12 months	64,798	253,005
Estimated Fair Value - 12 months or greater	1,949,631	1,210,148
Gross Unrealized Losses - 12 months or greater	94,006	108,194
Estimated Fair Value	7,267,089	9,537,991
Gross Unrealized Losses	158,804	361,199
Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	13,158	212,899
Gross Unrealized Losses - Less than 12 months	741	7,851
Estimated Fair Value - 12 months or greater	84,189	28,630
Gross Unrealized Losses - 12 months or greater	3,665	5,807
Estimated Fair Value	97,347	241,529
Gross Unrealized Losses	4,406	13,658
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	182,161	374,831
Gross Unrealized Losses - Less than 12 months	3,945	10,607
Estimated Fair Value - 12 months or greater	116,460	215,908
Gross Unrealized Losses - 12 months or greater	24,542	45,409
Estimated Fair Value	298,621	590,739
Gross Unrealized Losses	28,487	56,016
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	5,135,297	7,953,012
Gross Unrealized Losses - Less than 12 months	60,853	242,398
Estimated Fair Value - 12 months or greater	1,833,171	994,240
Gross Unrealized Losses - 12 months or greater	69,464	62,785
Estimated Fair Value	6,968,468	8,947,252
Gross Unrealized Losses	130,317	305,183
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	170,023	330,757
Gross Unrealized Losses - Less than 12 months	3,780	7,914
Estimated Fair Value - 12 months or greater	95,089	163,152
Gross Unrealized Losses - 12 months or greater	23,785	41,605
Estimated Fair Value	265,112	493,909
Gross Unrealized Losses	27,565	49,519
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	2,035,856	4,661,706
Gross Unrealized Losses - Less than 12 months	15,923	124,444
Estimated Fair Value - 12 months or greater	1,213,026	549,273
Gross Unrealized Losses - 12 months or greater	49,422	43,282
Estimated Fair Value	3,248,882	5,210,979
Gross Unrealized Losses	65,345	167,726
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	97,100	101,578
Gross Unrealized Losses - Less than 12 months	1,320	3,541
Estimated Fair Value - 12 months or greater	46,663	0
Gross Unrealized Losses - 12 months or greater	1,112	0
Estimated Fair Value	143,763	101,578
Gross Unrealized Losses	2,432	3,541
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	0
Gross Unrealized Losses - Less than 12 months	0	0
Estimated Fair Value - 12 months or greater	93	412
Gross Unrealized Losses - 12 months or greater	3	12
Estimated Fair Value	93	412
Gross Unrealized Losses	3	12
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	507,372	490,473
Gross Unrealized Losses - Less than 12 months	5,084	9,733
Estimated Fair Value - 12 months or greater	154,469	112,216
Gross Unrealized Losses - 12 months or greater	3,189	3,635
Estimated Fair Value	661,841	602,689
Gross Unrealized Losses	8,273	13,368
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	0	5,904
Gross Unrealized Losses - Less than 12 months	0	700
Estimated Fair Value - 12 months or greater	5,611	12,581
Gross Unrealized Losses - 12 months or greater	532	1,465
Estimated Fair Value	5,611	18,485
Gross Unrealized Losses	532	2,165
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	516,613	563,259
Gross Unrealized Losses - Less than 12 months	2,318	12,010
Estimated Fair Value - 12 months or greater	133,848	257,166
Gross Unrealized Losses - 12 months or greater	2,213	9,653
Estimated Fair Value	650,461	820,425
Gross Unrealized Losses	4,531	21,663
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		5,815
Gross Unrealized Losses - Less than 12 months		735
Estimated Fair Value - 12 months or greater		0
Gross Unrealized Losses - 12 months or greater		0
Estimated Fair Value		5,815
Gross Unrealized Losses		735
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	293,834	368,465
Gross Unrealized Losses - Less than 12 months	3,224	6,858
Estimated Fair Value - 12 months or greater	61,707	10,853
Gross Unrealized Losses - 12 months or greater	2,221	166
Estimated Fair Value	355,541	379,318
Gross Unrealized Losses	5,445	7,024
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	1,302,732	1,056,101
Gross Unrealized Losses - Less than 12 months	29,712	63,044
Estimated Fair Value - 12 months or greater	56,595	0
Gross Unrealized Losses - 12 months or greater	1,286	0
Estimated Fair Value	1,359,327	1,056,101
Gross Unrealized Losses	30,998	63,044
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	919
Gross Unrealized Losses - Less than 12 months	88
Estimated Fair Value - 12 months or greater	0
Gross Unrealized Losses - 12 months or greater	0
Estimated Fair Value	919
Gross Unrealized Losses	88
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	53,977	187,194
Gross Unrealized Losses - Less than 12 months	743	9,396
Estimated Fair Value - 12 months or greater	62,530	13,635
Gross Unrealized Losses - 12 months or greater	5,156	3,068
Estimated Fair Value	116,507	200,829
Gross Unrealized Losses	5,899	12,464
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	11,219	32,355
Gross Unrealized Losses - Less than 12 months	77	1,258
Estimated Fair Value - 12 months or greater	15,667	39,763
Gross Unrealized Losses - 12 months or greater	222	2,327
Estimated Fair Value	26,886	72,118
Gross Unrealized Losses	299	3,585
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	327,813	524,236
Gross Unrealized Losses - Less than 12 months	2,529	13,372
Estimated Fair Value - 12 months or greater	104,333	51,097
Gross Unrealized Losses - 12 months or greater	4,865	2,981
Estimated Fair Value	432,146	575,333
Gross Unrealized Losses	7,394	16,353
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	6,712	10,831
Gross Unrealized Losses - Less than 12 months	345	831
Estimated Fair Value - 12 months or greater	25,983	21,879
Gross Unrealized Losses - 12 months or greater	2,944	5,506
Estimated Fair Value	32,695	32,710
Gross Unrealized Losses	3,289	6,337
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	6,446	202,068
Gross Unrealized Losses - Less than 12 months	396	7,020
Estimated Fair Value - 12 months or greater	58,206	6,751
Gross Unrealized Losses - 12 months or greater	721	301
Estimated Fair Value	64,652	208,819
Gross Unrealized Losses	$ 1,117	$ 7,321