Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 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		$ 4,565,273
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		40,239
Estimated Fair Value - 12 months or greater		2,738,393
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		73,107
Estimated Fair Value		7,303,666
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss		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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		1,024
Estimated Fair Value - 12 months or greater		73,722
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		2,623
Estimated Fair Value		79,624
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss		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		225,474
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		3,450
Estimated Fair Value - 12 months or greater		93,723
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		7,559
Estimated Fair Value		319,197
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss		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	$ 10,738,781	4,339,799
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	226,619	36,789
Estimated Fair Value - 12 months or greater	2,440,830	2,644,670
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	115,895	65,548
Estimated Fair Value	13,179,611	6,984,469
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	342,514	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	452,086	194,879
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	29,597	3,317
Estimated Fair Value - 12 months or greater	46,976	75,731
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	8,337	6,933
Estimated Fair Value	499,062	270,610
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	37,934	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	7,282,079	1,886,212
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	155,541	17,099
Estimated Fair Value - 12 months or greater	883,699	1,009,750
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	46,908	28,080
Estimated Fair Value	8,165,778	2,895,962
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	202,449	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	1,900	1,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	114	20
Estimated Fair Value - 12 months or greater	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	0
Estimated Fair Value	1,900	1,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	114	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	39,192	18,688
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	390	91
Estimated Fair Value - 12 months or greater	109,552	111,560
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	2,536	1,596
Estimated Fair Value	148,744	130,248
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	2,926	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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	0
Estimated Fair Value - 12 months or greater	1,272	1,369
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	25	22
Estimated Fair Value	1,272	1,369
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	25	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	897,255	566,699
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	19,899	5,852
Estimated Fair Value - 12 months or greater	225,288	224,439
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	7,575	6,004
Estimated Fair Value	1,122,543	791,138
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	27,474	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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	0
Estimated Fair Value - 12 months or greater	1,173	1,489
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	47	53
Estimated Fair Value	1,173	1,489
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	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	612,053	434,274
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	7,001	2,707
Estimated Fair Value - 12 months or greater	114,605	168,524
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	2,423	2,434
Estimated Fair Value	726,658	602,798
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	9,424	5,141
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	57,385
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	662
Estimated Fair Value - 12 months or greater	12,045
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	169
Estimated Fair Value	69,430
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	831
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	538,464	220,401
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	8,026	1,914
Estimated Fair Value - 12 months or greater	103,447	103,269
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	4,210	2,920
Estimated Fair Value	641,911	323,670
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	12,236	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	488,095	800,298
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	16,856	6,177
Estimated Fair Value - 12 months or greater	744,844	767,197
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	42,558	15,756
Estimated Fair Value	1,232,939	1,567,495
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	59,414	21,933
US States and Political Subdivisions 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	511,371
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	30,373
Estimated Fair Value - 12 months or greater	61,466
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	8,578
Estimated Fair Value	572,837
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	38,951
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	135,158	43,510
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	3,331	242
Estimated Fair Value - 12 months or greater	67,949	68,666
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	4,685	4,054
Estimated Fair Value	203,107	112,176
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	8,016	4,296
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	11,250,152	28,600
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	256,992	113
Estimated Fair Value - 12 months or greater	2,502,296	15,134
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	124,473	551
Estimated Fair Value	13,752,448	43,734
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	381,465	664
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	746,485	369,717
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	15,575	2,707
Estimated Fair Value - 12 months or greater	191,446	191,265
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	5,000	4,704
Estimated Fair Value	937,931	560,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 20,575	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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		1
Estimated Fair Value - 12 months or greater		26,471
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		2,225
Estimated Fair Value		26,553
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss		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
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		1,023
Estimated Fair Value - 12 months or greater		47,251
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		398
Estimated Fair Value		53,071
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss		$ 1,421