Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 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	$ 6,362,753	$ 8,327,843
Gross Unrealized Losses - Less than 12 months	111,152	253,005
Estimated Fair Value - 12 months or greater	881,805	1,210,148
Gross Unrealized Losses - 12 months or greater	61,291	108,194
Estimated Fair Value	7,244,558	9,537,991
Gross Unrealized Losses	172,443	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	64,990	212,899
Gross Unrealized Losses - Less than 12 months	3,617	7,851
Estimated Fair Value - 12 months or greater	24,807	28,630
Gross Unrealized Losses - 12 months or greater	3,021	5,807
Estimated Fair Value	89,797	241,529
Gross Unrealized Losses	6,638	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	295,307	374,831
Gross Unrealized Losses - Less than 12 months	4,837	10,607
Estimated Fair Value - 12 months or greater	118,570	215,908
Gross Unrealized Losses - 12 months or greater	27,347	45,409
Estimated Fair Value	413,877	590,739
Gross Unrealized Losses	32,184	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	6,067,446	7,953,012
Gross Unrealized Losses - Less than 12 months	106,315	242,398
Estimated Fair Value - 12 months or greater	763,235	994,240
Gross Unrealized Losses - 12 months or greater	33,944	62,785
Estimated Fair Value	6,830,681	8,947,252
Gross Unrealized Losses	140,259	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	255,991	330,757
Gross Unrealized Losses - Less than 12 months	4,547	7,914
Estimated Fair Value - 12 months or greater	93,562	163,152
Gross Unrealized Losses - 12 months or greater	26,081	41,605
Estimated Fair Value	349,553	493,909
Gross Unrealized Losses	30,628	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	2,895,605	4,661,706
Gross Unrealized Losses - Less than 12 months	46,389	124,444
Estimated Fair Value - 12 months or greater	399,546	549,273
Gross Unrealized Losses - 12 months or greater	23,252	43,282
Estimated Fair Value	3,295,151	5,210,979
Gross Unrealized Losses	69,641	167,726
Canadian and Canadian provincial governments | 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	1,247
Gross Unrealized Losses - Less than 12 months	3
Estimated Fair Value - 12 months or greater	0
Gross Unrealized Losses - 12 months or greater	0
Estimated Fair Value	1,247
Gross Unrealized Losses	3
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	116,719	101,578
Gross Unrealized Losses - Less than 12 months	2,457	3,541
Estimated Fair Value - 12 months or greater	0	0
Gross Unrealized Losses - 12 months or greater	0	0
Estimated Fair Value	116,719	101,578
Gross Unrealized Losses	2,457	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	107	412
Gross Unrealized Losses - 12 months or greater	4	12
Estimated Fair Value	107	412
Gross Unrealized Losses	4	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	471,933	490,473
Gross Unrealized Losses - Less than 12 months	6,872	9,733
Estimated Fair Value - 12 months or greater	100,785	112,216
Gross Unrealized Losses - 12 months or greater	1,918	3,635
Estimated Fair Value	572,718	602,689
Gross Unrealized Losses	8,790	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	7,295	12,581
Gross Unrealized Losses - 12 months or greater	557	1,465
Estimated Fair Value	7,295	18,485
Gross Unrealized Losses	557	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	285,211	563,259
Gross Unrealized Losses - Less than 12 months	1,451	12,010
Estimated Fair Value - 12 months or greater	204,154	257,166
Gross Unrealized Losses - 12 months or greater	3,916	9,653
Estimated Fair Value	489,365	820,425
Gross Unrealized Losses	5,367	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	352,867	368,465
Gross Unrealized Losses - Less than 12 months	4,897	6,858
Estimated Fair Value - 12 months or greater	2,195	10,853
Gross Unrealized Losses - 12 months or greater	38	166
Estimated Fair Value	355,062	379,318
Gross Unrealized Losses	4,935	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	1,378,976	1,056,101
Gross Unrealized Losses - Less than 12 months	31,962	63,044
Estimated Fair Value - 12 months or greater	13,763	0
Gross Unrealized Losses - 12 months or greater	86	0
Estimated Fair Value	1,392,739	1,056,101
Gross Unrealized Losses	32,048	63,044
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	125,465	187,194
Gross Unrealized Losses - Less than 12 months	5,098	9,396
Estimated Fair Value - 12 months or greater	13,558	13,635
Gross Unrealized Losses - 12 months or greater	3,118	3,068
Estimated Fair Value	139,023	200,829
Gross Unrealized Losses	8,216	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	38,069	32,355
Gross Unrealized Losses - Less than 12 months	287	1,258
Estimated Fair Value - 12 months or greater	17,606	39,763
Gross Unrealized Losses - 12 months or greater	705	2,327
Estimated Fair Value	55,675	72,118
Gross Unrealized Losses	992	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	440,670	524,236
Gross Unrealized Losses - Less than 12 months	7,189	13,372
Estimated Fair Value - 12 months or greater	29,234	51,097
Gross Unrealized Losses - 12 months or greater	1,616	2,981
Estimated Fair Value	469,904	575,333
Gross Unrealized Losses	8,805	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	0	10,831
Gross Unrealized Losses - Less than 12 months	0	831
Estimated Fair Value - 12 months or greater	24,807	21,879
Gross Unrealized Losses - 12 months or greater	3,021	5,506
Estimated Fair Value	24,807	32,710
Gross Unrealized Losses	3,021	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	64,990	202,068
Gross Unrealized Losses - Less than 12 months	3,617	7,020
Estimated Fair Value - 12 months or greater	0	6,751
Gross Unrealized Losses - 12 months or greater	0	301
Estimated Fair Value	64,990	208,819
Gross Unrealized Losses	$ 3,617	$ 7,321