Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturity securities - USD ($) $ in Thousands	Jun. 30, 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	$ 2,286,525	$ 12,073,860
Gross Unrealized Losses - Less than 12 months	39,708	408,581
Estimated Fair Value - 12 months or greater	2,330,244	7,226,996
Gross Unrealized Losses - 12 months or greater	56,673	339,909
Estimated Fair Value	4,616,769	19,300,856
Gross Unrealized Losses	96,381	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	222,948	884,847
Gross Unrealized Losses - Less than 12 months	22,225	48,368
Estimated Fair Value - 12 months or greater	158,958	133,897
Gross Unrealized Losses - 12 months or greater	12,560	15,297
Estimated Fair Value	381,906	1,018,744
Gross Unrealized Losses	34,785	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	2,063,577	11,189,013
Gross Unrealized Losses - Less than 12 months	17,483	360,213
Estimated Fair Value - 12 months or greater	2,171,286	7,093,099
Gross Unrealized Losses - 12 months or greater	44,113	324,612
Estimated Fair Value	4,234,863	18,282,112
Gross Unrealized Losses	61,596	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	216,611	755,679
Gross Unrealized Losses - Less than 12 months	22,148	42,760
Estimated Fair Value - 12 months or greater	139,318	122,559
Gross Unrealized Losses - 12 months or greater	10,959	14,646
Estimated Fair Value	355,929	878,238
Gross Unrealized Losses	33,107	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	780,802	8,505,371
Gross Unrealized Losses - Less than 12 months	8,757	302,604
Estimated Fair Value - 12 months or greater	1,276,130	3,611,266
Gross Unrealized Losses - 12 months or greater	28,924	195,082
Estimated Fair Value	2,056,932	12,116,637
Gross Unrealized Losses	37,681	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		443
Gross Unrealized Losses - Less than 12 months		34
Estimated Fair Value - 12 months or greater		1,770
Gross Unrealized Losses - 12 months or greater		243
Estimated Fair Value		2,213
Gross Unrealized Losses		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	2,535	25,169
Gross Unrealized Losses - Less than 12 months	2	419
Estimated Fair Value - 12 months or greater	17,555	131,806
Gross Unrealized Losses - 12 months or greater	131	1,612
Estimated Fair Value	20,090	156,975
Gross Unrealized Losses	133	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
Gross Unrealized Losses - Less than 12 months	0	0
Estimated Fair Value - 12 months or greater	976	1,026
Gross Unrealized Losses - 12 months or greater	11	34
Estimated Fair Value	976	1,026
Gross Unrealized Losses	11	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	57,572	269,558
Gross Unrealized Losses - Less than 12 months	489	2,488
Estimated Fair Value - 12 months or greater	240,046	836,741
Gross Unrealized Losses - 12 months or greater	2,577	22,760
Estimated Fair Value	297,618	1,106,299
Gross Unrealized Losses	3,066	25,248
Asset-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
Gross Unrealized Losses - Less than 12 months		0
Estimated Fair Value - 12 months or greater		1,063
Gross Unrealized Losses - 12 months or greater		35
Estimated Fair Value		1,063
Gross Unrealized Losses		35
Asset-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	753,026	1,102,677
Gross Unrealized Losses - Less than 12 months	5,383	24,271
Estimated Fair Value - 12 months or greater	399,818	381,609
Gross Unrealized Losses - 12 months or greater	7,679	8,523
Estimated Fair Value	1,152,844	1,484,286
Gross Unrealized Losses	13,062	32,794
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	79,372	384,259
Gross Unrealized Losses - Less than 12 months	240	4,304
Estimated Fair Value - 12 months or greater	37,442	414,719
Gross Unrealized Losses - 12 months or greater	274	13,930
Estimated Fair Value	116,814	798,978
Gross Unrealized Losses	514	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	257,328	8,616
Gross Unrealized Losses - Less than 12 months	36	80
Estimated Fair Value - 12 months or greater	76,098	1,086,694
Gross Unrealized Losses - 12 months or greater	334	57,787
Estimated Fair Value	333,426	1,095,310
Gross Unrealized Losses	370	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	42,876	103,504
Gross Unrealized Losses - Less than 12 months	11	1,538
Estimated Fair Value - 12 months or greater	13,856	157,330
Gross Unrealized Losses - 12 months or greater	1,497	7,472
Estimated Fair Value	56,732	260,834
Gross Unrealized Losses	1,508	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	6,337	128,725
Gross Unrealized Losses - Less than 12 months	77	5,574
Estimated Fair Value - 12 months or greater	18,664	7,479
Gross Unrealized Losses - 12 months or greater	1,590	339
Estimated Fair Value	25,001	136,204
Gross Unrealized Losses	1,667	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	90,066	789,859
Gross Unrealized Losses - Less than 12 months	2,565	24,509
Estimated Fair Value - 12 months or greater	110,341	472,934
Gross Unrealized Losses - 12 months or greater	2,697	17,446
Estimated Fair Value	200,407	1,262,793
Gross Unrealized Losses	$ 5,262	$ 41,955