Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 4,565,273	$ 8,327,843
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	40,239	253,005
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,738,393	1,210,148
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	73,107	108,194
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	7,303,666	9,537,991
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	113,346	361,199
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	4,339,799	7,953,012
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	36,789	242,398
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,644,670	994,240
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	65,548	62,785
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	6,984,469	8,947,252
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	102,337	305,183
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,886,212	4,661,706
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	17,099	124,444
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,009,750	549,273
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	28,080	43,282
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,895,962	5,210,979
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	45,179	167,726
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Canadian Provincial Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	18,688	101,578
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	91	3,541
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	111,560	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	1,596	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	130,248	101,578
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	1,687	3,541
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	566,699	490,473
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	5,852	9,733
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	224,439	112,216
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	6,004	3,635
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	791,138	602,689
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	11,856	13,368
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	434,274	563,259
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	2,707	12,010
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	168,524	257,166
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	2,434	9,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	602,798	820,425
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	5,141	21,663
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	220,401	368,465
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1,914	6,858
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	103,269	10,853
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	2,920	166
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	323,670	379,318
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	4,834	7,024
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	800,298	1,056,101
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	6,177	63,044
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	767,197	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	15,756	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,567,495	1,056,101
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	21,933	63,044
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	43,510	187,194
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	242	9,396
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	68,666	13,635
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	4,054	3,068
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	112,176	200,829
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	4,296	12,464
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	369,717	524,236
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	2,707	13,372
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	191,265	51,097
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	4,704	2,981
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	560,982	575,333
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	7,411	16,353
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	225,474	374,831
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	3,450	10,607
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	93,723	215,908
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	7,559	45,409
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	319,197	590,739
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	11,009	56,016
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	194,879	330,757
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	3,317	7,914
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	75,731	163,152
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	6,933	41,605
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	270,610	493,909
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	10,250	49,519
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Canadian Provincial Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	20
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	20
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,369	412
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	22	12
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,369	412
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	22	12
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	5,904
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	0	700
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,489	12,581
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	53	1,465
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,489	18,485
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	53	2,165
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		5,815
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses		735
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		5,815
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses		735
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	28,600	32,355
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	113	1,258
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	15,134	39,763
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	551	2,327
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	43,734	72,118
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	664	3,585
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	5,902	212,899
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1,024	7,851
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	73,722	28,630
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	2,623	5,807
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	79,624	241,529
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	3,647	13,658
Equity Securities [Member] | Nonredeemable Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	82	10,831
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1	831
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	26,471	21,879
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	2,225	5,506
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	26,553	32,710
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	2,226	6,337
Equity Securities [Member] | Equity Securities Other Than Non-redeemable Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	5,820	202,068
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1,023	7,020
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	47,251	6,751
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	398	301
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	53,071	208,819
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 1,421	$ 7,321