Fair Value of Assets and Liabilities (Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	$ 32,093,625	$ 29,642,905
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	(22,529)	(76,698)
Cash and Cash Equivalents, Fair Value Disclosure	338,601	406,521
Fair Value Measured On Recurring Basis Short Term Investments	44,241	530,773
Other Assets, Fair Value Disclosure	26,958	14,996
Investments, Fair Value Disclosure	705,625	587,757
Total	33,186,521	31,106,254
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	990,308	1,070,584
Other Liabilities, Fair Value Disclosure		1,095,409
Total	1,011,851
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,933,479	1,757,449
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	0	0
Cash and Cash Equivalents, Fair Value Disclosure	338,601	406,521
Fair Value Measured On Recurring Basis Short Term Investments	8,276	524,946
Other Assets, Fair Value Disclosure	0	0
Investments, Fair Value Disclosure	462,482	323,638
Total	2,742,838	3,012,554
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Other Liabilities, Fair Value Disclosure		0
Total	0
Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	27,932,189	25,460,690
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	0	0
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Fair Value Measured On Recurring Basis Short Term Investments	32,619	5,827
Other Assets, Fair Value Disclosure	0	0
Investments, Fair Value Disclosure	243,143	264,119
Total	28,207,951	25,730,636
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Other Liabilities, Fair Value Disclosure		22,206
Total	19,081
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	2,227,957	2,424,766
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	(22,529)	(76,698)
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Fair Value Measured On Recurring Basis Short Term Investments	3,346	0
Other Assets, Fair Value Disclosure	26,958	14,996
Investments, Fair Value Disclosure	0	0
Total	2,235,732	2,363,064
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	990,308	1,070,584
Other Liabilities, Fair Value Disclosure		1,073,203
Total	992,770
Nonredeemable Preferred Stock [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	51,123	87,520
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	38,317	81,809
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	12,806	5,711
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Equity Securities Other Than Non-redeemable Preferred Stock [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	224,238	38,342
Equity Securities Other Than Non-redeemable Preferred Stock [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	224,238	38,342
Equity Securities Other Than Non-redeemable Preferred Stock [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Equity Securities Other Than Non-redeemable Preferred Stock [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Other [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	11,036	8,170
Other [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	11,036	8,170
Other [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Other [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Interest Rate Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	93,508	71,882
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	24,374	20,989
Interest Rate Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Interest Rate Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	93,508	71,882
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	24,374	20,989
Interest Rate Swap [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Foreign Exchange Forward [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		20
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	5,070	6,744
Foreign Exchange Forward [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Foreign Exchange Forward [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		20
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	5,070	6,744
Foreign Exchange Forward [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Consumer Price Index Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		(292)
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	262
Consumer Price Index Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0
Consumer Price Index Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		(292)
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	262
Consumer Price Index Swap [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0
Credit Default Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	9,136	2,567
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(5)	5,390
Credit Default Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Credit Default Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	9,136	2,567
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(5)	5,390
Credit Default Swap [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Equity Option [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	26,070	40,644
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(7,389)	(6,009)
Equity Option [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Equity Option [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	26,070	40,644
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(7,389)	(6,009)
Equity Option [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Currency Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	100,394	141,357
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(3,231)	(4,908)
Currency Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Currency Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	100,394	141,357
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(3,231)	(4,908)
Currency Swap [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	190,120	197,547
Fair Value Option Contractholder Directed Unit Linked Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	188,891	195,317
Fair Value Option Contractholder Directed Unit Linked Investments [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	1,229	2,230
Fair Value Option Contractholder Directed Unit Linked Investments [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Mortality Swap [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	2,462	2,619
Mortality Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Mortality Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	0	0
Mortality Swap [Member] | Fair Value, Inputs, Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	2,462	2,619
Corporate Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	19,619,084	17,708,156
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	310,995	269,039
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	18,035,836	16,212,147
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,272,253	1,226,970
Canadian Provincial Governments [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	3,644,046	3,576,759
Canadian Provincial Governments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	0	0
Canadian Provincial Governments [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	3,168,081	3,160,683
Canadian Provincial Governments [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	475,965	416,076
Residential Mortgage Backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,278,576	1,311,477
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	0	0
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,118,285	980,828
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	160,291	330,649
Asset-backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,429,344	1,212,676
Asset-backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	0	0
Asset-backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,210,064	908,840
Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	219,280	303,836
Commercial Mortgage Backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,363,654	1,483,087
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	0	0
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,342,509	1,414,524
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	21,145	68,563
US Treasury and Government [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,468,302	1,381,659
US Treasury and Government [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,345,755	1,227,858
US Treasury and Government [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	98,059	127,536
US Treasury and Government [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	24,488	26,265
US States and Political Subdivisions Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	591,796	511,014
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	0	0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	550,130	472,672
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	41,666	38,342
Foreign Government Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	2,698,823	2,458,077
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	276,729	260,552
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	2,409,225	2,183,460
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	$ 12,869	$ 14,065