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	Mar. 31, 2019	Dec. 31, 2018
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 41,738,443	$ 39,992,346
Equity Method Investments, Fair Value Disclosure	89,865	82,197
Funds withheld at interest embedded derivatives	107,506	109,597
Cash and cash equivalents	855,265	485,167
Short-term investments	75,360	105,991
Investments, Fair Value Disclosure	335,260	378,469
Other Assets, Fair Value Disclosure	48,869	47,789
Assets, Fair Value Disclosure	43,250,568	41,201,556
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	903,935	944,865
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	931,967	972,314
Interest Rate Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	48,683	37,976
Liabilities Fair Value Disclosure
Other liabilities	(17,620)	(18,542)
Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	33,113	50,740
Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(20,505)	(11,384)
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	202,848	197,770
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(6,201)	4,466
Liabilities Fair Value Disclosure
Other liabilities	(12,757)	(371)
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	18,676	36,206
Liabilities Fair Value Disclosure
Other liabilities	(1,476)	(6,630)
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	38,141	51,311
Liabilities Fair Value Disclosure
Other liabilities	(4,629)	(4,155)
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	489	(369)
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	25,077,338	23,982,119
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,277,864	3,892,385
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,994,186	1,869,221
Asset-backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,241,986	2,149,204
Assets, Fair Value Disclosure	81,417	77,842
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,431,746	1,419,034
US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,567,563	2,185,874
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	792,717	752,194
Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,355,043	3,742,315
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,464,139	2,067,529
Equity Method Investments, Fair Value Disclosure	49,443	48,737
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	853,927	473,509
Short-term investments	4,766	4,989
Investments, Fair Value Disclosure	201,833	196,781
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	2,574,108	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 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 | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	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	201,833	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 [Member]
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 Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,464,139	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	37,982,991	35,930,086
Equity Method Investments, Fair Value Disclosure	0	0
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	1,319	11,658
Short-term investments	41,392	98,774
Investments, Fair Value Disclosure	133,427	181,688
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	38,159,129	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	28,521	27,080
Fair Value Inputs Level 2 Member | Interest Rate Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	48,683	37,976
Liabilities Fair Value Disclosure
Other liabilities	(17,620)	(18,542)
Fair Value Inputs Level 2 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	33,113	50,740
Fair Value Inputs Level 2 Member | Consumer Price Index Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(20,505)	(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	1,015	989
Fair Value Inputs Level 2 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(6,201)	4,466
Liabilities Fair Value Disclosure
Other liabilities	(12,757)	(371)
Fair Value Inputs Level 2 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	18,676	36,206
Liabilities Fair Value Disclosure
Other liabilities	(1,476)	(6,630)
Fair Value Inputs Level 2 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	38,141	51,311
Liabilities Fair Value Disclosure
Other liabilities	(4,629)	(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	23,546,583	22,651,194
Fair Value Inputs Level 2 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,670,073	3,364,261
Fair Value Inputs Level 2 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,987,664	1,862,366
Fair Value Inputs Level 2 Member | Asset-backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,128,199	2,053,632
Fair Value Inputs Level 2 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,431,725	1,419,012
Fair Value Inputs Level 2 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	86,215	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	782,555	741,992
Fair Value Inputs Level 2 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,349,977	3,737,309
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,291,313	1,994,731
Equity Method Investments, Fair Value Disclosure	40,422	33,460
Funds withheld at interest embedded derivatives	107,506	109,597
Cash and cash equivalents	19	0
Short-term investments	29,202	2,228
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	48,869	47,789
Assets, Fair Value Disclosure	2,517,331	2,187,805
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	903,935	944,865
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	903,446	945,234
Fair Value Inputs Level 3 Member | Interest Rate 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 | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	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	489	(369)
Fair Value Inputs Level 3 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,530,755	1,330,925
Fair Value Inputs Level 3 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	607,791	528,124
Fair Value Inputs Level 3 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	6,522	6,855
Fair Value Inputs Level 3 Member | Asset-backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	113,787	95,572
Fair Value Inputs Level 3 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	21	22
Fair Value Inputs Level 3 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	17,209	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	10,162	10,202
Fair Value Inputs Level 3 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 5,066	$ 5,006