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, 2016	Dec. 31, 2015
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	$ 8,327,843	$ 10,578,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	253,005	513,764
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,210,148	822,840
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	108,194	105,483
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	9,537,991	11,401,822
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	361,199	619,247
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	7,953,012	9,930,955
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	242,398	434,506
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	994,240	658,939
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	62,785	60,939
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	8,947,252	10,589,894
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	305,183	495,445
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	4,661,706	6,388,148
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	124,444	323,961
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	549,273	294,755
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	43,282	40,861
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	5,210,979	6,682,903
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	167,726	364,822
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	101,578	122,746
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	3,541	2,532
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	101,578	122,746
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	3,541	2,532
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	490,473	452,297
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	9,733	7,036
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	112,216	82,314
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	3,635	4,057
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	602,689	534,611
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	13,368	11,093
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	563,259	581,701
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	12,010	9,825
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	257,166	199,298
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	9,653	7,100
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	820,425	780,999
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	21,663	16,925
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	368,465	514,877
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	6,858	9,806
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,853	31,177
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	166	997
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	379,318	546,054
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	7,024	10,803
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	1,056,101	1,010,387
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	63,044	57,718
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,056,101	1,010,387
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	63,044	57,718
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	187,194	157,837
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	9,396	5,349
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	13,635	13,016
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	3,068	3,718
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	200,829	170,853
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	12,464	9,067
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	524,236	702,962
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	13,372	18,279
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	51,097	38,379
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	2,981	4,206
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	575,333	741,341
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	16,353	22,485
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	374,831	648,027
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	10,607	79,258
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	215,908	163,901
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	45,409	44,544
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	590,739	811,928
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	56,016	123,802
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	330,757	554,688
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	7,914	71,171
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	163,152	114,427
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	41,605	31,076
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	493,909	669,115
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	49,519	102,247
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	22,646
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	0	282
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	412	7,679
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	12	298
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	412	30,325
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	12	580
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	5,904	6,772
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	700	201
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	12,581	9,335
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	1,465	1,250
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	18,485	16,107
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	2,165	1,451
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	3,253
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	735	248
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	767
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	117
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	5,815	4,020
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	735	365
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	32,355	60,668
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1,258	7,356
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	39,763	31,693
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	2,327	11,803
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	72,118	92,361
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	3,585	19,159
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	212,899	50,658
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	7,851	4,417
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	28,630	12,191
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	5,807	3,787
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	241,529	62,849
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	13,658	8,204
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	10,831	12,331
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	831	2,175
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	21,879	12,191
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	5,506	3,787
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	32,710	24,522
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	6,337	5,962
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	202,068	38,327
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	7,020	2,242
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	6,751	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	301	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	208,819	38,327
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 7,321	$ 2,242