Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 49,481,267	$ 39,992,346
Equity Method Investments, Fair Value Disclosure	134,453	82,197
Funds withheld at interest embedded derivatives	121,275	109,597
Cash and cash equivalents	927,175	485,167
Short-term investments	89,130	105,991
Investments, Fair Value Disclosure	394,376	378,469
Other Assets, Fair Value Disclosure	51,620	47,789
Assets, Fair Value Disclosure	51,199,296	41,201,556
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	989,170	944,865
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	1,037,260	972,314
Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	73,085	37,976
Liabilities Fair Value Disclosure
Other liabilities	(26,024)	(18,542)
Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(12,454)
Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	43,654	50,740
Liabilities Fair Value Disclosure
Other liabilities	124
Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(32,885)	(11,384)
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	252,277	197,770
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(7,174)	4,466
Liabilities Fair Value Disclosure
Other liabilities	12,923	371
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,150	36,206
Liabilities Fair Value Disclosure
Other liabilities	18,694	6,630
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	26,384	51,311
Liabilities Fair Value Disclosure
Other liabilities	(7,615)	(4,155)
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(853)	(369)
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	30,353,219	23,982,119
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,633,219	3,892,385
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,439,842	1,869,221
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,822,635	2,149,204
Assets, Fair Value Disclosure	102,027	77,842
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,782,218	1,419,034
US Government Agencies Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,668,505	2,185,874
Assets, Fair Value Disclosure	17,141	18,025
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,190,103	752,194
Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,591,526	3,742,315
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,541,294	2,067,529
Equity Method Investments, Fair Value Disclosure	86,968	48,737
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	922,291	473,509
Short-term investments	17,375	4,989
Investments, Fair Value Disclosure	197,195	196,781
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	2,765,123	2,791,545
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	0	0
Fair Value Inputs Level 1 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0
Fair Value Inputs Level 1 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0
Fair Value Inputs Level 1 Member | Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	197,195	196,781
Fair Value Inputs Level 1 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | US Government Agencies Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,541,294	2,067,529
Fair Value Inputs Level 1 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	45,244,476	35,930,086
Equity Method Investments, Fair Value Disclosure	0	0
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	4,884	11,658
Short-term investments	68,898	98,774
Investments, Fair Value Disclosure	197,181	181,688
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	45,515,439	36,222,206
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	47,237	27,080
Fair Value Inputs Level 2 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	73,085	37,976
Liabilities Fair Value Disclosure
Other liabilities	(26,024)	(18,542)
Fair Value Inputs Level 2 Member | Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(12,454)
Fair Value Inputs Level 2 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	43,654	50,740
Liabilities Fair Value Disclosure
Other liabilities	124
Fair Value Inputs Level 2 Member | Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(32,885)	(11,384)
Fair Value Inputs Level 2 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	55,082	989
Fair Value Inputs Level 2 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(7,174)	4,466
Liabilities Fair Value Disclosure
Other liabilities	12,923	371
Fair Value Inputs Level 2 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,150	36,206
Liabilities Fair Value Disclosure
Other liabilities	18,694	6,630
Fair Value Inputs Level 2 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	26,384	51,311
Liabilities Fair Value Disclosure
Other liabilities	(7,615)	(4,155)
Fair Value Inputs Level 2 Member | Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 2 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	28,547,959	22,651,194
Fair Value Inputs Level 2 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,942,376	3,364,261
Fair Value Inputs Level 2 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,416,762	1,862,366
Fair Value Inputs Level 2 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,719,526	2,053,632
Fair Value Inputs Level 2 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,752,250	1,419,012
Fair Value Inputs Level 2 Member | US Government Agencies Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	110,070	100,320
Fair Value Inputs Level 2 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,180,150	741,992
Fair Value Inputs Level 2 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,575,383	3,737,309
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,695,497	1,994,731
Equity Method Investments, Fair Value Disclosure	47,485	33,460
Funds withheld at interest embedded derivatives	121,275	109,597
Cash and cash equivalents	0	0
Short-term investments	2,857	2,228
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	51,620	47,789
Assets, Fair Value Disclosure	2,918,734	2,187,805
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	989,170	944,865
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	990,023	945,234
Fair Value Inputs Level 3 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0
Fair Value Inputs Level 3 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0
Fair Value Inputs Level 3 Member | Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 3 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(853)	(369)
Fair Value Inputs Level 3 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,805,260	1,330,925
Fair Value Inputs Level 3 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	690,843	528,124
Fair Value Inputs Level 3 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	23,080	6,855
Fair Value Inputs Level 3 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	103,109	95,572
Fair Value Inputs Level 3 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	29,968	22
Fair Value Inputs Level 3 Member | US Government Agencies Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	17,141	18,025
Fair Value Inputs Level 3 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	9,953	10,202
Fair Value Inputs Level 3 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 16,143	$ 5,006