Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
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	$ 14,260,261	$ 4,565,273
Gross Unrealized Losses - Less than 12 months	454,493	40,239
Estimated Fair Value - 12 months or greater	2,494,303	2,738,393
Gross Unrealized Losses - 12 months or greater	137,715	73,107
Estimated Fair Value	16,754,564	7,303,666
Gross Unrealized Losses	592,208	113,346
Equity securities
Estimated Fair Values And Gross Unrealized Losses, OTTI Impairment Losses Reported In AOCI [Line Items]
Estimated Fair Value - Less than 12 months		5,902
Gross Unrealized Losses - Less than 12 months		1,024
Estimated Fair Value - 12 months or greater		73,722
Gross Unrealized Losses - 12 months or greater		2,623
Estimated Fair Value		79,624
Gross Unrealized Losses		3,647
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	883,576	225,474
Gross Unrealized Losses - Less than 12 months	55,107	3,450
Estimated Fair Value - 12 months or greater	66,027	93,723
Gross Unrealized Losses - 12 months or greater	10,277	7,559
Estimated Fair Value	949,603	319,197
Gross Unrealized Losses	65,384	11,009
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	13,376,685	4,339,799
Gross Unrealized Losses - Less than 12 months	399,386	36,789
Estimated Fair Value - 12 months or greater	2,428,276	2,644,670
Gross Unrealized Losses - 12 months or greater	127,438	65,548
Estimated Fair Value	15,804,961	6,984,469
Gross Unrealized Losses	526,824	102,337
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	735,338	194,879
Gross Unrealized Losses - Less than 12 months	47,846	3,317
Estimated Fair Value - 12 months or greater	56,042	75,731
Gross Unrealized Losses - 12 months or greater	9,851	6,933
Estimated Fair Value	791,380	270,610
Gross Unrealized Losses	57,697	10,250
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	9,135,620	1,886,212
Gross Unrealized Losses - Less than 12 months	299,909	17,099
Estimated Fair Value - 12 months or greater	818,555	1,009,750
Gross Unrealized Losses - 12 months or greater	51,938	28,080
Estimated Fair Value	9,954,175	2,895,962
Gross Unrealized Losses	351,847	45,179
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	1,864	1,995
Gross Unrealized Losses - Less than 12 months	150	20
Estimated Fair Value - 12 months or greater	0	0
Gross Unrealized Losses - 12 months or greater	0	0
Estimated Fair Value	1,864	1,995
Gross Unrealized Losses	150	20
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	46,392	18,688
Gross Unrealized Losses - Less than 12 months	668	91
Estimated Fair Value - 12 months or greater	110,326	111,560
Gross Unrealized Losses - 12 months or greater	3,058	1,596
Estimated Fair Value	156,718	130,248
Gross Unrealized Losses	3,726	1,687
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	1,194	1,369
Gross Unrealized Losses - 12 months or greater	24	22
Estimated Fair Value	1,194	1,369
Gross Unrealized Losses	24	22
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	1,101,941	566,699
Gross Unrealized Losses - Less than 12 months	25,546	5,852
Estimated Fair Value - 12 months or greater	241,914	224,439
Gross Unrealized Losses - 12 months or greater	9,032	6,004
Estimated Fair Value	1,343,855	791,138
Gross Unrealized Losses	34,578	11,856
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	0
Gross Unrealized Losses - Less than 12 months	0	0
Estimated Fair Value - 12 months or greater	1,148	1,489
Gross Unrealized Losses - 12 months or greater	47	53
Estimated Fair Value	1,148	1,489
Gross Unrealized Losses	47	53
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	807,714	434,274
Gross Unrealized Losses - Less than 12 months	10,637	2,707
Estimated Fair Value - 12 months or greater	139,676	168,524
Gross Unrealized Losses - 12 months or greater	3,187	2,434
Estimated Fair Value	947,390	602,798
Gross Unrealized Losses	13,824	5,141
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	612,214	220,401
Gross Unrealized Losses - Less than 12 months	11,097	1,914
Estimated Fair Value - 12 months or greater	104,426	103,269
Gross Unrealized Losses - 12 months or greater	4,601	2,920
Estimated Fair Value	716,640	323,670
Gross Unrealized Losses	15,698	4,834
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	584,758	800,298
Gross Unrealized Losses - Less than 12 months	20,707	6,177
Estimated Fair Value - 12 months or greater	747,679	767,197
Gross Unrealized Losses - 12 months or greater	45,958	15,756
Estimated Fair Value	1,332,437	1,567,495
Gross Unrealized Losses	66,665	21,933
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	168,817	43,510
Gross Unrealized Losses - Less than 12 months	5,064	242
Estimated Fair Value - 12 months or greater	66,122	68,666
Gross Unrealized Losses - 12 months or greater	4,257	4,054
Estimated Fair Value	234,939	112,176
Gross Unrealized Losses	9,321	4,296
Debt Security, Government, Non-US [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	146,374	28,600
Gross Unrealized Losses - Less than 12 months	7,111	113
Estimated Fair Value - 12 months or greater	7,643	15,134
Gross Unrealized Losses - 12 months or greater	355	551
Estimated Fair Value	154,017	43,734
Gross Unrealized Losses	7,466	664
Debt Security, Government, Non-US [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	919,229	369,717
Gross Unrealized Losses - Less than 12 months	25,758	2,707
Estimated Fair Value - 12 months or greater	199,578	191,265
Gross Unrealized Losses - 12 months or greater	5,407	4,704
Estimated Fair Value	1,118,807	560,982
Gross Unrealized Losses	$ 31,165	7,411
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		82
Gross Unrealized Losses - Less than 12 months		1
Estimated Fair Value - 12 months or greater		26,471
Gross Unrealized Losses - 12 months or greater		2,225
Estimated Fair Value		26,553
Gross Unrealized Losses		2,226
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		5,820
Gross Unrealized Losses - Less than 12 months		1,023
Estimated Fair Value - 12 months or greater		47,251
Gross Unrealized Losses - 12 months or greater		398
Estimated Fair Value		53,071
Gross Unrealized Losses		$ 1,421