Investments (Investments in Fixed Maturity and Equity Securities Available-for-Sale by Sector) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 24,006,407	$ 21,966,803
Available-for-sale Securities	23,982,119	23,210,968
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	38,882,168	35,281,412
Available-for-sale Securities, Gross Unrealized Gains	1,858,668	2,982,754
Available-for-sale Securities, Gross Unrealized Losses	748,490	113,346
Available-for-sale Securities	$ 39,992,346	$ 38,150,820
Percentage Of Total Available For Sale Securities	100.00%	100.00%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 275	$ 275
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		102,841
Available-for-sale Securities, Gross Unrealized Gains		958
Available-for-sale Securities, Gross Unrealized Losses		3,647
Available-for-sale Securities		$ 100,152
Percentage Of Total Available For Sale Securities		100.00%
Corporate Debt Securities [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	24,006,407	$ 21,966,803
Available-for-sale Securities, Gross Unrealized Gains	530,804	1,299,594
Available-for-sale Securities, Gross Unrealized Losses	555,092	55,429
Available-for-sale Securities	$ 23,982,119	$ 23,210,968
Percentage Of Total Available For Sale Securities	59.90%	60.90%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Canadian Provincial Governments [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,768,466	2,843,273
Available-for-sale Securities, Gross Unrealized Gains	1,126,227	1,378,510
Available-for-sale Securities, Gross Unrealized Losses	2,308	1,707
Available-for-sale Securities	$ 3,892,385	$ 4,220,076
Percentage Of Total Available For Sale Securities	9.70%	11.10%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Residential Mortgage Backed Securities [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,872,236	1,695,126
Available-for-sale Securities, Gross Unrealized Gains	22,267	36,632
Available-for-sale Securities, Gross Unrealized Losses	25,282	11,878
Available-for-sale Securities	$ 1,869,221	$ 1,719,880
Percentage Of Total Available For Sale Securities	4.70%	4.50%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Asset-backed Securities [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,171,254	1,634,758
Available-for-sale Securities, Gross Unrealized Gains	10,779	18,798
Available-for-sale Securities, Gross Unrealized Losses	32,829	5,194
Available-for-sale Securities	$ 2,149,204	$ 1,648,362
Percentage Of Total Available For Sale Securities	5.40%	4.30%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 275	$ 275
Commercial Mortgage Backed Securities [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,428,115	1,285,594
Available-for-sale Securities, Gross Unrealized Gains	9,153	22,627
Available-for-sale Securities, Gross Unrealized Losses	18,234	4,834
Available-for-sale Securities	$ 1,419,034	$ 1,303,387
Percentage Of Total Available For Sale Securities	3.50%	3.40%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
US Treasury and Government [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,233,537	1,953,436
Available-for-sale Securities, Gross Unrealized Gains	10,204	12,089
Available-for-sale Securities, Gross Unrealized Losses	57,867	21,933
Available-for-sale Securities	$ 2,185,874	$ 1,943,592
Percentage Of Total Available For Sale Securities	5.50%	5.10%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
US States and Political Subdivisions Debt Securities [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	721,290	647,727
Available-for-sale Securities, Gross Unrealized Gains	39,914	59,997
Available-for-sale Securities, Gross Unrealized Losses	9,010	4,296
Available-for-sale Securities	$ 752,194	$ 703,428
Percentage Of Total Available For Sale Securities	1.90%	1.80%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Debt Security, Government, Non-US [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	3,680,863	3,254,695
Available-for-sale Securities, Gross Unrealized Gains	109,320	154,507
Available-for-sale Securities, Gross Unrealized Losses	47,868	8,075
Available-for-sale Securities	$ 3,742,315	$ 3,401,127
Percentage Of Total Available For Sale Securities	9.40%	8.90%
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities	$ 0	$ 0
Nonredeemable Preferred Stock [Member] | Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		41,553
Available-for-sale Securities, Gross Unrealized Gains		479
Available-for-sale Securities, Gross Unrealized Losses		2,226
Available-for-sale Securities		$ 39,806
Percentage Of Total Available For Sale Securities		39.70%
Equity Securities Other Than Non-redeemable Preferred Stock [Member] | Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 61,288
Available-for-sale Securities, Gross Unrealized Gains		479
Available-for-sale Securities, Gross Unrealized Losses		1,421
Available-for-sale Securities		$ 60,346
Percentage Of Total Available For Sale Securities		60.30%