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	Jun. 30, 2019	Dec. 31, 2018
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 46,189,305	$ 39,992,346
Equity Method Investments, Fair Value Disclosure	146,755	82,197
Funds withheld at interest embedded derivatives	112,767	109,597
Cash and cash equivalents	755,129	485,167
Short-term investments	126,468	105,991
Investments, Fair Value Disclosure	377,658	378,469
Other Assets, Fair Value Disclosure	51,755	47,789
Assets, Fair Value Disclosure	47,759,837	41,201,556
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	908,491	944,865
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	954,407	972,314
Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	61,298	37,976
Liabilities Fair Value Disclosure
Other liabilities	(19,945)	(18,542)
Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	27,532	50,740
Liabilities Fair Value Disclosure
Other liabilities	(653)
Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(25,292)	(11,384)
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	255,174	197,770
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(6,696)	4,466
Liabilities Fair Value Disclosure
Other liabilities	12,752	371
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	14,375	36,206
Liabilities Fair Value Disclosure
Other liabilities	269	6,630
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	25,975	51,311
Liabilities Fair Value Disclosure
Other liabilities	(12,194)	(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	27,635,431	23,982,119
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,573,963	3,892,385
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,276,515	1,869,221
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,494,664	2,149,204
Assets, Fair Value Disclosure	97,055	77,842
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,747,285	1,419,034
US Government Agencies Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,778,180	2,185,874
Assets, Fair Value Disclosure	17,406	18,025
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,303,574	752,194
Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,379,693	3,742,315
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,651,425	2,067,529
Equity Method Investments, Fair Value Disclosure	101,948	48,737
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	737,286	473,509
Short-term investments	17,963	4,989
Investments, Fair Value Disclosure	205,805	196,781
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	2,714,427	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 | 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	205,805	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	1,651,425	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	42,053,169	35,930,086
Equity Method Investments, Fair Value Disclosure	0	0
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	17,843	11,658
Short-term investments	80,118	98,774
Investments, Fair Value Disclosure	171,853	181,688
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	42,322,983	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	45,063	27,080
Fair Value Inputs Level 2 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	61,298	37,976
Liabilities Fair Value Disclosure
Other liabilities	(19,945)	(18,542)
Fair Value Inputs Level 2 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	27,532	50,740
Liabilities Fair Value Disclosure
Other liabilities	(653)
Fair Value Inputs Level 2 Member | Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(25,292)	(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	49,369	989
Fair Value Inputs Level 2 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(6,696)	4,466
Liabilities Fair Value Disclosure
Other liabilities	12,752	371
Fair Value Inputs Level 2 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	14,375	36,206
Liabilities Fair Value Disclosure
Other liabilities	269	6,630
Fair Value Inputs Level 2 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	25,975	51,311
Liabilities Fair Value Disclosure
Other liabilities	(12,194)	(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	25,991,067	22,651,194
Fair Value Inputs Level 2 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,902,936	3,364,261
Fair Value Inputs Level 2 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,260,483	1,862,366
Fair Value Inputs Level 2 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,384,342	2,053,632
Fair Value Inputs Level 2 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,747,266	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	109,349	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	1,293,719	741,992
Fair Value Inputs Level 2 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,364,007	3,737,309
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,484,711	1,994,731
Equity Method Investments, Fair Value Disclosure	44,807	33,460
Funds withheld at interest embedded derivatives	112,767	109,597
Cash and cash equivalents	0	0
Short-term investments	28,387	2,228
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	51,755	47,789
Assets, Fair Value Disclosure	2,722,427	2,187,805
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	908,491	944,865
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	909,344	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 | 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	1,644,364	1,330,925
Fair Value Inputs Level 3 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	671,027	528,124
Fair Value Inputs Level 3 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	16,032	6,855
Fair Value Inputs Level 3 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	110,322	95,572
Fair Value Inputs Level 3 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	19	22
Fair Value Inputs Level 3 Member | US Government Agencies Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	17,406	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	9,855	10,202
Fair Value Inputs Level 3 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 15,686	$ 5,006