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, 2015	Dec. 31, 2014
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	$ 29,642,905	$ 25,480,972
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	(76,698)	22,094
Cash and Cash Equivalents, Fair Value Disclosure	406,521	899,846
Fair Value Measured On Recurring Basis Short Term Investments	530,773	45,190
Other Assets, Fair Value Disclosure	14,996	7,727
Investments, Fair Value Disclosure	587,757	490,590
Total	31,106,254	26,946,419
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,070,584	1,085,166
Other Liabilities, Fair Value Disclosure		1,127,720
Total	1,095,409
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,757,449	838,946
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	0	0
Cash and Cash Equivalents, Fair Value Disclosure	406,521	899,846
Fair Value Measured On Recurring Basis Short Term Investments	524,946	21,536
Other Assets, Fair Value Disclosure	0	0
Investments, Fair Value Disclosure	323,638	260,116
Total	3,012,554	2,020,444
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	25,460,690	22,392,896
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	5,827	23,654
Other Assets, Fair Value Disclosure	0	0
Investments, Fair Value Disclosure	264,119	222,570
Total	25,730,636	22,639,120
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Other Liabilities, Fair Value Disclosure		41,757
Total	22,206
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	2,424,766	2,249,130
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	(76,698)	22,094
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Fair Value Measured On Recurring Basis Short Term Investments	0	0
Other Assets, Fair Value Disclosure	14,996	7,727
Investments, Fair Value Disclosure	0	7,904
Total	2,363,064	2,286,855
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,070,584	1,085,166
Other Liabilities, Fair Value Disclosure		1,085,963
Total	1,073,203
Nonredeemable Preferred Stock [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	87,520	99,363
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	81,809	91,450
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	5,711	9
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	7,904
Equity Securities Other Than Non-redeemable Preferred Stock [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	38,342	27,497
Equity Securities Other Than Non-redeemable Preferred Stock [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	38,342	27,497
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	8,170	6,420
Other [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	8,170	6,420
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	71,882	84,578
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	20,989	12,957
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	71,882	84,578
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	20,989	12,957
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
Interest Rate Options [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		18,195
Interest Rate Options [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Interest Rate Options [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		18,195
Interest Rate Options [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Foreign Exchange Forward [Member]
Assets, Fair Value Disclosure [Abstract]
Other Assets, Fair Value Disclosure	20
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	6,744	15,011
Foreign Exchange Forward [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Other Assets, 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]
Other Assets, Fair Value Disclosure	20
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	6,744	15,011
Foreign Exchange Forward [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Other Assets, 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)	(561)
Consumer Price Index Swap [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Consumer Price Index Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	(292)	(561)
Consumer Price Index Swap [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	0
Credit Default Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	2,567	8,606
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	5,390	419
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	2,567	8,606
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	5,390	419
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	40,644	35,242
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(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
Equity Option [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	40,644	35,242
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(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
Currency Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	141,357	70,906
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(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
Currency Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	141,357	70,906
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(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
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	197,547	140,344
Fair Value Option Contractholder Directed Unit Linked Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	195,317	134,749
Fair Value Option Contractholder Directed Unit Linked Investments [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	2,230	5,595
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
Purchase Commitment [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure		13,370
Purchase Commitment [Member] | Fair Value, Inputs, Level 1 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure		0
Purchase Commitment [Member] | Fair Value, Inputs, Level 2 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure		13,370
Purchase Commitment [Member] | Fair Value, Inputs, Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure		0
Mortality Swap [Member]
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	2,619	797
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	2,619	797
Corporate Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	17,708,156	14,885,583
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	269,039	115,822
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	16,212,147	13,459,334
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,226,970	1,310,427
Canadian Provincial Governments [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	3,576,759	3,865,265
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	3,160,683	3,865,265
Canadian Provincial Governments [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	416,076	0
Residential Mortgage Backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,311,477	1,037,896
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	980,828	849,802
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	330,649	188,094
Asset-backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,212,676	1,069,586
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	908,840	496,626
Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	303,836	572,960
Commercial Mortgage Backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,483,087	1,532,591
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	1,414,524	1,445,845
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	68,563	86,746
US Treasury and Government [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,381,659	525,851
US Treasury and Government [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,227,858	437,129
US Treasury and Government [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	127,536	60,193
US Treasury and Government [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	26,265	28,529
US States and Political Subdivisions Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	511,014	426,076
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	472,672	383,365
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	38,342	42,711
Foreign Government Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	2,458,077	2,138,124
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	260,552	285,995
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	2,183,460	1,832,466
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	$ 14,065	$ 19,663