Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturity securities - USD ($) $ in Thousands	Sep. 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	$ 3,039,667	$ 12,073,860
Gross Unrealized Losses - Less than 12 months	60,114	408,581
Estimated Fair Value - 12 months or greater	1,251,213	7,226,996
Gross Unrealized Losses - 12 months or greater	42,129	339,909
Estimated Fair Value	4,290,880	19,300,856
Gross Unrealized Losses	102,243	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	220,064	884,847
Gross Unrealized Losses - Less than 12 months	24,862	48,368
Estimated Fair Value - 12 months or greater	136,782	133,897
Gross Unrealized Losses - 12 months or greater	17,305	15,297
Estimated Fair Value	356,846	1,018,744
Gross Unrealized Losses	42,167	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	2,819,603	11,189,013
Gross Unrealized Losses - Less than 12 months	35,252	360,213
Estimated Fair Value - 12 months or greater	1,114,431	7,093,099
Gross Unrealized Losses - 12 months or greater	24,824	324,612
Estimated Fair Value	3,934,034	18,282,112
Gross Unrealized Losses	60,076	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	206,108	755,679
Gross Unrealized Losses - Less than 12 months	24,561	42,760
Estimated Fair Value - 12 months or greater	122,670	122,559
Gross Unrealized Losses - 12 months or greater	16,417	14,646
Estimated Fair Value	328,778	878,238
Gross Unrealized Losses	40,978	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	1,570,113	8,505,371
Gross Unrealized Losses - Less than 12 months	25,598	302,604
Estimated Fair Value - 12 months or greater	410,462	3,611,266
Gross Unrealized Losses - 12 months or greater	11,576	195,082
Estimated Fair Value	1,980,575	12,116,637
Gross Unrealized Losses	37,174	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	867	25,169
Gross Unrealized Losses - Less than 12 months	10	419
Estimated Fair Value - 12 months or greater	17,509	131,806
Gross Unrealized Losses - 12 months or greater	180	1,612
Estimated Fair Value	18,376	156,975
Gross Unrealized Losses	190	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	939	1,026
Gross Unrealized Losses - 12 months or greater	11	34
Estimated Fair Value	939	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	222,644	269,558
Gross Unrealized Losses - Less than 12 months	987	2,488
Estimated Fair Value - 12 months or greater	116,037	836,741
Gross Unrealized Losses - 12 months or greater	1,218	22,760
Estimated Fair Value	338,681	1,106,299
Gross Unrealized Losses	2,205	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	765,223	1,102,677
Gross Unrealized Losses - Less than 12 months	4,861	24,271
Estimated Fair Value - 12 months or greater	413,297	381,609
Gross Unrealized Losses - 12 months or greater	9,133	8,523
Estimated Fair Value	1,178,520	1,484,286
Gross Unrealized Losses	13,994	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	1,042
Gross Unrealized Losses - Less than 12 months	11
Estimated Fair Value - 12 months or greater	0
Gross Unrealized Losses - 12 months or greater	0
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	48,699	384,259
Gross Unrealized Losses - Less than 12 months	218	4,304
Estimated Fair Value - 12 months or greater	32,638	414,719
Gross Unrealized Losses - 12 months or greater	232	13,930
Estimated Fair Value	81,337	798,978
Gross Unrealized Losses	450	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	2,758	8,616
Gross Unrealized Losses - Less than 12 months	7	80
Estimated Fair Value - 12 months or greater	65,220	1,086,694
Gross Unrealized Losses - 12 months or greater	221	57,787
Estimated Fair Value	67,978	1,095,310
Gross Unrealized Losses	228	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	11,129	103,504
Gross Unrealized Losses - Less than 12 months	97	1,538
Estimated Fair Value - 12 months or greater	13,838	157,330
Gross Unrealized Losses - 12 months or greater	1,503	7,472
Estimated Fair Value	24,967	260,834
Gross Unrealized Losses	1,600	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	12,914	128,725
Gross Unrealized Losses - Less than 12 months	290	5,574
Estimated Fair Value - 12 months or greater	13,173	7,479
Gross Unrealized Losses - 12 months or greater	877	339
Estimated Fair Value	26,087	136,204
Gross Unrealized Losses	1,167	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	198,170	789,859
Gross Unrealized Losses - Less than 12 months	3,474	24,509
Estimated Fair Value - 12 months or greater	45,430	472,934
Gross Unrealized Losses - 12 months or greater	761	17,446
Estimated Fair Value	243,600	1,262,793
Gross Unrealized Losses	$ 4,235	$ 41,955