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, 2016	Dec. 31, 2015
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 31,148,714	$ 29,642,905
Funds withheld at interest embedded derivatives	(168,948)	(76,698)
Cash and cash equivalents	577,884	406,521
Short-term investments	364,214	530,773
Investments, Fair Value Disclosure	632,230	587,757
Other Assets, Fair Value Disclosure	15,806	14,996
Assets, Fair Value Disclosure, Recurring	32,569,900	31,106,254
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,118,069	1,070,584
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	1,134,692	1,095,409
Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	78,085	87,520
Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	39,791	38,342
Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	29,270	8,170
Interest Rate Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	131,665	71,882
Liabilities Fair Value Disclosure
Other liabilities	12,410	20,989
Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	915
Other Assets, Fair Value Disclosure		20
Liabilities Fair Value Disclosure
Other liabilities	3,312	6,744
Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(402)	(292)
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	202,130	197,547
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	4,162	2,567
Liabilities Fair Value Disclosure
Other liabilities	3,177	5,390
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	40,096	40,644
Liabilities Fair Value Disclosure
Other liabilities	(7,004)	(6,009)
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	106,518	141,357
Liabilities Fair Value Disclosure
Other liabilities	1,685	(4,908)
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	3,043	2,619
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	18,512,048	17,708,156
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,881,117	3,576,759
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,380,897	1,311,477
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,289,381	1,212,676
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,508,183	1,483,087
US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,507,080	1,381,659
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	552,243	511,014
Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,517,765	2,458,077
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,870,866	1,757,449
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	577,884	406,521
Short-term investments	353,227	524,946
Investments, Fair Value Disclosure	332,573	323,638
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	3,134,550	3,012,554
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	62,668	81,809
Fair Value Inputs Level 1 Member \| Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	39,791	38,342
Fair Value Inputs Level 1 Member \| Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	29,270	8,170
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
Other Assets, 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	0
Fair Value Inputs Level 1 Member \| Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	200,844	195,317
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	265,592	269,039
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	1,349,361	1,227,858
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	255,913	260,552
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	26,790,318	25,460,690
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	0	0
Short-term investments	10,987	5,827
Investments, Fair Value Disclosure	299,657	264,119
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	27,100,962	25,730,636
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	13,580	22,206
Fair Value Inputs Level 2 Member \| Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	15,417	5,711
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	0
Fair Value Inputs Level 2 Member \| Interest Rate Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	131,665	71,882
Liabilities Fair Value Disclosure
Other liabilities	12,410	20,989
Fair Value Inputs Level 2 Member \| Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	915
Other Assets, Fair Value Disclosure		20
Liabilities Fair Value Disclosure
Other liabilities	3,312	6,744
Fair Value Inputs Level 2 Member \| Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(402)	(292)
Fair Value Inputs Level 2 Member \| Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,286	2,230
Fair Value Inputs Level 2 Member \| Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	4,162	2,567
Liabilities Fair Value Disclosure
Other liabilities	3,177	5,390
Fair Value Inputs Level 2 Member \| Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	40,096	40,644
Liabilities Fair Value Disclosure
Other liabilities	(7,004)	(6,009)
Fair Value Inputs Level 2 Member \| Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	106,518	141,357
Liabilities Fair Value Disclosure
Other liabilities	1,685	(4,908)
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	17,002,796	16,212,147
Fair Value Inputs Level 2 Member \| Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,393,734	3,160,683
Fair Value Inputs Level 2 Member \| Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,047,644	980,828
Fair Value Inputs Level 2 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,004,161	908,840
Fair Value Inputs Level 2 Member \| Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,444,609	1,414,524
Fair Value Inputs Level 2 Member \| US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	131,839	127,536
Fair Value Inputs Level 2 Member \| US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	517,619	472,672
Fair Value Inputs Level 2 Member \| Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,247,916	2,183,460
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,487,530	2,424,766
Funds withheld at interest embedded derivatives	(168,948)	(76,698)
Cash and cash equivalents	0	0
Short-term investments	0	0
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	15,806	14,996
Assets, Fair Value Disclosure, Recurring	2,334,388	2,363,064
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,118,069	1,070,584
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	1,121,112	1,073,203
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	0
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
Other Assets, 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	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	3,043	2,619
Fair Value Inputs Level 3 Member \| Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,243,660	1,226,970
Fair Value Inputs Level 3 Member \| Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	487,383	416,076
Fair Value Inputs Level 3 Member \| Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	333,253	330,649
Fair Value Inputs Level 3 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	285,220	303,836
Fair Value Inputs Level 3 Member \| Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	63,574	68,563
Fair Value Inputs Level 3 Member \| US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	25,880	26,265
Fair Value Inputs Level 3 Member \| US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	34,624	38,342
Fair Value Inputs Level 3 Member \| Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 13,936	$ 14,065

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