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, 2017	Dec. 31, 2016
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 36,381,742	$ 32,093,625
Funds withheld at interest embedded derivatives	84,325	(22,529)
Cash and cash equivalents	332,186	338,601
Short-term investments	56,454	44,241
Investments, Fair Value Disclosure	456,996	705,625
Other Assets, Fair Value Disclosure	37,827	26,958
Assets, Fair Value Disclosure, Recurring	37,349,530	33,186,521
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	989,940	990,308
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	1,011,846	1,011,851
Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	38,901	51,123
Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	74,030	224,238
Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		11,036
Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	46,258	93,508
Liabilities Fair Value Disclosure
Other liabilities	8,953	24,374
Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,418
Liabilities Fair Value Disclosure
Other liabilities	10,287	5,070
Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	230
Liabilities Fair Value Disclosure
Other liabilities	61	262
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	210,660	190,120
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,903	9,136
Liabilities Fair Value Disclosure
Other liabilities	512	5
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	9,700	26,070
Liabilities Fair Value Disclosure
Other liabilities	10,306	7,389
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	68,896	100,394
Liabilities Fair Value Disclosure
Other liabilities	11,740	3,231
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	1,683	2,462
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	22,535,830	19,619,084
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,991,185	3,644,046
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,679,305	1,278,576
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,694,568	1,429,344
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,313,322	1,363,654
US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,603,669	1,468,302
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	661,031	591,796
Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,902,832	2,698,823
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,414,937	1,933,479
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	332,186	338,601
Short-term investments	0	8,276
Investments, Fair Value Disclosure	322,067	462,482
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	3,069,190	2,742,838
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	0	0
Fair Value Inputs Level 1 Member | Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	38,901	38,317
Fair Value Inputs Level 1 Member | Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	74,030	224,238
Fair Value Inputs Level 1 Member | Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		11,036
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
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0
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	209,136	188,891
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	615,433	310,995
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 Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,480,357	1,345,755
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 | Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	319,147	276,729
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	31,833,890	27,932,189
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	0	0
Short-term investments	53,095	32,619
Investments, Fair Value Disclosure	134,929	243,143
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	32,021,914	28,207,951
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	20,223	19,081
Fair Value Inputs Level 2 Member | Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	12,806
Fair Value Inputs Level 2 Member | Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 2 Member | Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		0
Fair Value Inputs Level 2 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	46,258	93,508
Liabilities Fair Value Disclosure
Other liabilities	8,953	24,374
Fair Value Inputs Level 2 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,418
Liabilities Fair Value Disclosure
Other liabilities	10,287	5,070
Fair Value Inputs Level 2 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	230
Liabilities Fair Value Disclosure
Other liabilities	61	262
Fair Value Inputs Level 2 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,524	1,229
Fair Value Inputs Level 2 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,903	9,136
Liabilities Fair Value Disclosure
Other liabilities	512	5
Fair Value Inputs Level 2 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	9,700	26,070
Liabilities Fair Value Disclosure
Other liabilities	10,306	7,389
Fair Value Inputs Level 2 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	68,896	100,394
Liabilities Fair Value Disclosure
Other liabilities	11,740	3,231
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	20,648,377	18,035,836
Fair Value Inputs Level 2 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,465,820	3,168,081
Fair Value Inputs Level 2 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,561,310	1,118,285
Fair Value Inputs Level 2 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,549,371	1,210,064
Fair Value Inputs Level 2 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,311,375	1,342,509
Fair Value Inputs Level 2 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	100,061	98,059
Fair Value Inputs Level 2 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	619,519	550,130
Fair Value Inputs Level 2 Member | Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,578,057	2,409,225
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,132,915	2,227,957
Funds withheld at interest embedded derivatives	84,325	(22,529)
Cash and cash equivalents	0	0
Short-term investments	3,359	3,346
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	37,827	26,958
Assets, Fair Value Disclosure, Recurring	2,258,426	2,235,732
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	989,940	990,308
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	991,623	992,770
Fair Value Inputs Level 3 Member | Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 3 Member | Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 3 Member | Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		0
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
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0
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	1,683	2,462
Fair Value Inputs Level 3 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,272,020	1,272,253
Fair Value Inputs Level 3 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	525,365	475,965
Fair Value Inputs Level 3 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	117,995	160,291
Fair Value Inputs Level 3 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	145,197	219,280
Fair Value Inputs Level 3 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,947	21,145
Fair Value Inputs Level 3 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	23,251	24,488
Fair Value Inputs Level 3 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	41,512	41,666
Fair Value Inputs Level 3 Member | Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 5,628	$ 12,869