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, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	$ 38,150,820	$ 32,093,625
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	122,194	(22,529)
Cash and Cash Equivalents, Fair Value Disclosure	356,788	338,601
Fair Value Measured On Recurring Basis Short Term Investments	50,746	44,241
Other Assets, Fair Value Disclosure	40,659	26,958
Investments, Fair Value Disclosure	456,306	705,625
Total	39,177,513	33,186,521
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,014,228	990,308
Other Liabilities, Fair Value Disclosure		(1,011,851)
Total	1,038,469
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,818,006	1,933,479
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	0	0
Cash and Cash Equivalents, Fair Value Disclosure	354,071	338,601
Fair Value Measured On Recurring Basis Short Term Investments	0	8,276
Other Assets, Fair Value Disclosure	0	0
Investments, Fair Value Disclosure	317,770	462,482
Total	2,489,847	2,742,838
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	34,098,204	27,932,189
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	0	0
Cash and Cash Equivalents, Fair Value Disclosure	2,717	0
Fair Value Measured On Recurring Basis Short Term Investments	47,650	32,619
Other Assets, Fair Value Disclosure	0	0
Investments, Fair Value Disclosure	138,536	243,143
Total	34,287,107	28,207,951
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Other Liabilities, Fair Value Disclosure		(19,081)
Total	22,558
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	2,234,610	2,227,957
Fair Value Measured On Recurring Basis Funds Held Under Reinsurance Agreements Asset Embedded Derivatives	122,194	(22,529)
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Fair Value Measured On Recurring Basis Short Term Investments	3,096	3,346
Other Assets, Fair Value Disclosure	40,659	26,958
Investments, Fair Value Disclosure	0	0
Total	2,400,559	2,235,732
Liabilities, Fair Value Disclosure [Abstract]
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,014,228	990,308
Other Liabilities, Fair Value Disclosure		(992,770)
Total	1,015,911
Nonredeemable Preferred Stock [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	39,806	51,123
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	39,806	38,317
Nonredeemable Preferred Stock [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	0	12,806
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	60,346	224,238
Equity Securities Other Than Non-redeemable Preferred Stock [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	60,346	224,238
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
Other [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		11,036
Other [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Other [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure		0
Interest Rate Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	51,359	93,508
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(14,311)	(24,374)
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	51,359	93,508
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(14,311)	(24,374)
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]
Other Assets, Fair Value Disclosure	730
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(7,213)	(5,070)
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	730
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(7,213)	(5,070)
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	(221)
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(1,939)	(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	0
Consumer Price Index Swap [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	(221)
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(1,939)	(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	0
Credit Default Swap [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	5,908	9,136
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(760)	(5)
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	5,908	9,136
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(760)	(5)
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	16,932	26,070
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(6,339)	(7,389)
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	16,932	26,070
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(6,339)	(7,389)
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	62,905	100,394
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(6,194)	(3,231)
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	62,905	100,394
Liabilities, Fair Value Disclosure [Abstract]
Other Liabilities, Fair Value Disclosure	(6,194)	(3,231)
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	218,541	190,120
Fair Value Option Contractholder Directed Unit Linked Investments [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	217,618	188,891
Fair Value Option Contractholder Directed Unit Linked Investments [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Investments, Fair Value Disclosure	923	1,229
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	(1,683)	(2,462)
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	(1,683)	(2,462)
Corporate Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	23,210,968	19,619,084
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	0	310,995
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	21,873,696	18,035,836
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,337,272	1,272,253
Canadian Provincial Governments [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	4,220,076	3,644,046
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	3,626,134	3,168,081
Canadian Provincial Governments [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	593,942	475,965
Residential Mortgage Backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,719,880	1,278,576
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	1,611,998	1,118,285
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	107,882	160,291
Asset-backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,648,362	1,429,344
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	1,524,888	1,210,064
Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	123,474	219,280
Commercial Mortgage Backed Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,303,387	1,363,654
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	1,300,153	1,342,509
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	3,234	21,145
US Treasury and Government [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,943,592	1,468,302
US Treasury and Government [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	1,818,006	1,345,755
US Treasury and Government [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	103,075	98,059
US Treasury and Government [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	22,511	24,488
US States and Political Subdivisions Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	703,428	591,796
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	662,225	550,130
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	41,203	41,666
Foreign Government Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	3,401,127	2,698,823
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	0	276,729
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	3,396,035	2,409,225
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Fixed maturity securities available-for-sale	$ 5,092	$ 12,869