Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 12,073,860	$ 4,565,273
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	408,581	40,239
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,226,996	2,738,393
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	339,909	73,107
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	19,300,856	7,303,666
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	748,490	113,346
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	11,189,013	4,339,799
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	360,213	36,789
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,093,099	2,644,670
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	324,612	65,548
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	18,282,112	6,984,469
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	684,825	102,337
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	8,505,371	1,886,212
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	302,604	17,099
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3,611,266	1,009,750
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	195,082	28,080
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	12,116,637	2,895,962
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	497,686	45,179
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Canadian Provincial Governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	25,169	18,688
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	419	91
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	131,806	111,560
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	1,612	1,596
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	156,975	130,248
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	2,031	1,687
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	269,558	566,699
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	2,488	5,852
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	836,741	224,439
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	22,760	6,004
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,106,299	791,138
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	25,248	11,856
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,102,677	434,274
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	24,271	2,707
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	381,609	168,524
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	8,523	2,434
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,484,286	602,798
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	32,794	5,141
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	384,259	220,401
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	4,304	1,914
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	414,719	103,269
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	13,930	2,920
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	798,978	323,670
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	18,234	4,834
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | US Government Agencies Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	8,616	800,298
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	80	6,177
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,086,694	767,197
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	57,787	15,756
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,095,310	1,567,495
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	57,867	21,933
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	103,504	43,510
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	1,538	242
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	157,330	68,666
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	7,472	4,054
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	260,834	112,176
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	9,010	4,296
Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member] | Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	789,859	369,717
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	24,509	2,707
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	472,934	191,265
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	17,446	4,704
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,262,793	560,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	41,955	7,411
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	884,847	225,474
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	48,368	3,450
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	133,897	93,723
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	15,297	7,559
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,018,744	319,197
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	63,665	11,009
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	755,679	194,879
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	42,760	3,317
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	122,559	75,731
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	14,646	6,933
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	878,238	270,610
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	57,406	10,250
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Canadian Provincial Governments [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	443	1,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	34	20
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,770	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	243	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,213	1,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	277	20
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [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	1,026	1,369
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	34	22
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,026	1,369
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	34	22
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [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	1,063	1,489
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	35	53
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,063	1,489
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	35	53
Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member] | Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	128,725	28,600
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	5,574	113
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,479	15,134
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	339	551
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	136,204	43,734
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 5,913	664
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		5,902
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses		1,024
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		73,722
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses		2,623
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		79,624
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses		3,647
Equity Securities [Member] | Nonredeemable Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		82
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses		1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		26,471
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses		2,225
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		26,553
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses		2,226
Equity Securities [Member] | Equity Securities Other Than Non-redeemable Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		5,820
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses		1,023
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		47,251
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses		398
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		53,071
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses		$ 1,421