Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturity securities - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	$ 2,439,315	$ 12,073,860
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	56,533	408,581
Estimated Fair Value - 12 months or greater	7,032,814	7,226,996
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	173,690	339,909
Estimated Fair Value	9,472,129	19,300,856
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	230,223	748,490
Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	242,781	884,847
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	22,236	48,368
Estimated Fair Value - 12 months or greater	191,246	133,897
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	12,351	15,297
Estimated Fair Value	434,027	1,018,744
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	34,587	63,665
Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	2,196,534	11,189,013
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	34,297	360,213
Estimated Fair Value - 12 months or greater	6,841,568	7,093,099
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	161,339	324,612
Estimated Fair Value	9,038,102	18,282,112
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	195,636	684,825
Corporate Debt Securities [Member] | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	215,368	755,679
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	21,398	42,760
Estimated Fair Value - 12 months or greater	161,915	122,559
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	10,685	14,646
Estimated Fair Value	377,283	878,238
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	32,083	57,406
Corporate Debt Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,140,753	8,505,371
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	23,578	302,604
Estimated Fair Value - 12 months or greater	3,756,371	3,611,266
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	108,434	195,082
Estimated Fair Value	4,897,124	12,116,637
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	132,012	497,686
Canadian and Canadian provincial governments | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	0	443
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	34
Estimated Fair Value - 12 months or greater	1,001	1,770
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	23	243
Estimated Fair Value	1,001	2,213
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	23	277
Canadian and Canadian provincial governments | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	2,812	25,169
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	19	419
Estimated Fair Value - 12 months or greater	72,244	131,806
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	636	1,612
Estimated Fair Value	75,056	156,975
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	655	2,031
Residential mortgage-backed securities | Non-investment grade 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
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Estimated Fair Value - 12 months or greater	1,029	1,026
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	31	34
Estimated Fair Value	1,029	1,026
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	31	34
Residential mortgage-backed securities | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	29,505	269,558
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	122	2,488
Estimated Fair Value - 12 months or greater	672,715	836,741
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	10,109	22,760
Estimated Fair Value	702,220	1,106,299
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	10,231	25,248
Asset-backed Securities [Member] | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Estimated Fair Value - 12 months or greater		1,063
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		35
Estimated Fair Value		1,063
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		35
Asset-backed Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	774,538	1,102,677
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7,246	24,271
Estimated Fair Value - 12 months or greater	528,169	381,609
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	8,472	8,523
Estimated Fair Value	1,302,707	1,484,286
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	15,718	32,794
Commercial mortgage-backed securities | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	27,413
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	838
Estimated Fair Value - 12 months or greater	27,301
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,612
Estimated Fair Value	54,714
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	2,450
Commercial mortgage-backed securities | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	100,664	384,259
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	369	4,304
Estimated Fair Value - 12 months or greater	227,404	414,719
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,746	13,930
Estimated Fair Value	328,068	798,978
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	3,115	18,234
US Government Agencies Debt Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	1,265	8,616
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	80
Estimated Fair Value - 12 months or greater	1,125,274	1,086,694
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	21,237	57,787
Estimated Fair Value	1,126,539	1,095,310
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	21,238	57,867
US States and Political Subdivisions Debt Securities [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	31,416	103,504
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	149	1,538
Estimated Fair Value - 12 months or greater	113,463	157,330
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,492	7,472
Estimated Fair Value	144,879	260,834
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	2,641	9,010
Debt Security, Government, Non-US [Member] | Non-investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months		128,725
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		5,574
Estimated Fair Value - 12 months or greater		7,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		339
Estimated Fair Value		136,204
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss		5,913
Debt Security, Government, Non-US [Member] | Investment grade securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months	115,581	789,859
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,813	24,509
Estimated Fair Value - 12 months or greater	345,928	472,934
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7,213	17,446
Estimated Fair Value	461,509	1,262,793
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ 10,026	$ 41,955