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, 2015	Dec. 31, 2014
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 27,411,788	$ 25,480,972
Funds withheld at interest embedded derivatives	(49,498)	22,094
Cash and cash equivalents	822,184	899,846
Short-term investments	11,829	45,190
Investments, Fair Value Disclosure	523,692	490,590
Other Assets, Fair Value Disclosure	13,298	7,727
Assets, Fair Value Disclosure, Recurring	28,733,293	26,946,419
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,106,410	1,085,166
Liabilities Fair Value Disclosure
Other liabilities		1,127,720
Liabilities, Fair Value Disclosure, Recurring	1,145,363
Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	84,709	99,363
Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	25,665	27,497
Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	8,289	6,420
Interest Rate Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	86,504	84,578
Liabilities Fair Value Disclosure
Other liabilities	25,241	12,957
Interest Rate Options
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		18,195
Foreign Currency Forwards
Assets Fair Value Disclosure
Other Assets, Fair Value Disclosure	343
Liabilities Fair Value Disclosure
Other liabilities	6,675	15,011
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	127,819	70,906
Liabilities Fair Value Disclosure
Other liabilities	1,761
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	129,511	140,344
Consumer Price Index Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(268)	(561)
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,088	8,606
Liabilities Fair Value Disclosure
Other liabilities	3,080	419
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	60,032	35,242
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	2,196	797
Purchase Commitment [Member]
Liabilities Fair Value Disclosure
Other liabilities		13,370
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	16,173,447	14,885,583
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,542,726	3,865,265
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,283,460	1,037,896
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,061,372	1,069,586
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,491,951	1,532,591
US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,317,218	525,851
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	501,503	426,076
Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,040,111	2,138,124
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,638,107	838,946
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	822,184	899,846
Short-term investments	4,422	21,536
Investments, Fair Value Disclosure	232,280	260,116
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	2,696,993	2,020,444
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Other liabilities		0
Liabilities, Fair Value Disclosure, Recurring	0
Fair Value Inputs Level 1 Member \| Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	72,717	91,450
Fair Value Inputs Level 1 Member \| Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	25,665	27,497
Fair Value Inputs Level 1 Member \| Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	8,289	6,420
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 \| Interest Rate Options
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		0
Fair Value Inputs Level 1 Member \| Foreign Currency Forwards
Assets Fair Value Disclosure
Other Assets, Fair Value Disclosure	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
Fair Value Inputs Level 1 Member \| Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	125,609	134,749
Fair Value Inputs Level 1 Member \| Consumer Price Index Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
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
Fair Value Inputs Level 1 Member \| Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member \| Purchase Commitment [Member]
Liabilities Fair Value Disclosure
Other liabilities		0
Fair Value Inputs Level 1 Member \| Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	214,549	115,822
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	1,162,453	437,129
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	261,105	285,995
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	23,398,550	22,392,896
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	0	0
Short-term investments	7,407	23,654
Investments, Fair Value Disclosure	279,420	222,570
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	23,685,377	22,639,120
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Other liabilities		41,757
Liabilities, Fair Value Disclosure, Recurring	36,757
Fair Value Inputs Level 2 Member \| Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	9
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	86,504	84,578
Liabilities Fair Value Disclosure
Other liabilities	25,241	12,957
Fair Value Inputs Level 2 Member \| Interest Rate Options
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		18,195
Fair Value Inputs Level 2 Member \| Foreign Currency Forwards
Assets Fair Value Disclosure
Other Assets, Fair Value Disclosure	343
Liabilities Fair Value Disclosure
Other liabilities	6,675	15,011
Fair Value Inputs Level 2 Member \| Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	127,819	70,906
Liabilities Fair Value Disclosure
Other liabilities	1,761
Fair Value Inputs Level 2 Member \| Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	3,902	5,595
Fair Value Inputs Level 2 Member \| Consumer Price Index Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(268)	(561)
Fair Value Inputs Level 2 Member \| Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,088	8,606
Liabilities Fair Value Disclosure
Other liabilities	3,080	419
Fair Value Inputs Level 2 Member \| Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	60,032	35,242
Fair Value Inputs Level 2 Member \| Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 2 Member \| Purchase Commitment [Member]
Liabilities Fair Value Disclosure
Other liabilities		13,370
Fair Value Inputs Level 2 Member \| Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	14,711,208	13,459,334
Fair Value Inputs Level 2 Member \| Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,542,726	3,865,265
Fair Value Inputs Level 2 Member \| Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	913,793	849,802
Fair Value Inputs Level 2 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	457,169	496,626
Fair Value Inputs Level 2 Member \| Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,419,308	1,445,845
Fair Value Inputs Level 2 Member \| US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	127,445	60,193
Fair Value Inputs Level 2 Member \| US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	462,509	383,365
Fair Value Inputs Level 2 Member \| Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,764,392	1,832,466
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,375,131	2,249,130
Funds withheld at interest embedded derivatives	(49,498)	22,094
Cash and cash equivalents	0	0
Short-term investments	0	0
Investments, Fair Value Disclosure	11,992	7,904
Other Assets, Fair Value Disclosure	13,298	7,727
Assets, Fair Value Disclosure, Recurring	2,350,923	2,286,855
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,106,410	1,085,166
Liabilities Fair Value Disclosure
Other liabilities		1,085,963
Liabilities, Fair Value Disclosure, Recurring	1,108,606
Fair Value Inputs Level 3 Member \| Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	11,992	7,904
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 \| Interest Rate Options
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		0
Fair Value Inputs Level 3 Member \| Foreign Currency Forwards
Assets Fair Value Disclosure
Other Assets, Fair Value Disclosure	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
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 \| Consumer Price Index Swaps
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
Fair Value Inputs Level 3 Member \| Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	2,196	797
Fair Value Inputs Level 3 Member \| Purchase Commitment [Member]
Liabilities Fair Value Disclosure
Other liabilities		0
Fair Value Inputs Level 3 Member \| Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,247,690	1,310,427
Fair Value Inputs Level 3 Member \| Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 3 Member \| Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	369,667	188,094
Fair Value Inputs Level 3 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	604,203	572,960
Fair Value Inputs Level 3 Member \| Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	72,643	86,746
Fair Value Inputs Level 3 Member \| US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	27,320	28,529
Fair Value Inputs Level 3 Member \| US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	38,994	42,711
Fair Value Inputs Level 3 Member \| Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 14,614	$ 19,663

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