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	Jun. 30, 2015	Dec. 31, 2014
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 28,063,975	$ 25,480,972
Funds withheld at interest embedded derivatives	(3,329)	22,094
Cash and cash equivalents	665,037	899,846
Short-term investments	34,733	45,190
Investments, Fair Value Disclosure	469,324	490,590
Other Assets, Fair Value Disclosure	10,853	7,727
Assets, Fair Value Disclosure, Recurring	29,240,593	26,946,419
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,069,154	1,085,166
Liabilities Fair Value Disclosure
Other liabilities		1,127,720
Liabilities, Fair Value Disclosure, Recurring	1,095,323
Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	90,574	99,363
Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	37,094	27,497
Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	9,852	6,420
Interest Rate Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	52,864	84,578
Liabilities Fair Value Disclosure
Other liabilities	16,152	12,957
Interest Rate Options
Assets Fair Value Disclosure
Investments, Fair Value Disclosure		18,195
Foreign Currency Forwards
Liabilities Fair Value Disclosure
Other liabilities	7,422	15,011
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	88,303	70,906
Liabilities Fair Value Disclosure
Other liabilities	413
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	137,529	140,344
Consumer Price Index Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(23)	(561)
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	8,248	8,606
Liabilities Fair Value Disclosure
Other liabilities	428	419
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	44,883	35,242
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	1,754	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	16,489,406	14,885,583
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,793,807	3,865,265
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,190,879	1,037,896
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,062,624	1,069,586
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,525,469	1,532,591
US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,263,998	525,851
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	506,172	426,076
Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,231,620	2,138,124
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,537,021	838,946
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	665,037	899,846
Short-term investments	20,896	21,536
Investments, Fair Value Disclosure	258,002	260,116
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	2,480,956	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	78,186	91,450
Fair Value Inputs Level 1 Member \| Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	37,094	27,497
Fair Value Inputs Level 1 Member \| Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	9,852	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
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	132,870	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	157,790	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	1,111,189	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	268,042	285,995
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	24,250,730	22,392,896
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	0	0
Short-term investments	13,837	23,654
Investments, Fair Value Disclosure	198,934	222,570
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	24,463,501	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	24,415
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	52,864	84,578
Liabilities Fair Value Disclosure
Other liabilities	16,152	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
Liabilities Fair Value Disclosure
Other liabilities	7,422	15,011
Fair Value Inputs Level 2 Member \| Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	88,303	70,906
Liabilities Fair Value Disclosure
Other liabilities	413
Fair Value Inputs Level 2 Member \| Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	4,659	5,595
Fair Value Inputs Level 2 Member \| Consumer Price Index Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(23)	(561)
Fair Value Inputs Level 2 Member \| Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	8,248	8,606
Liabilities Fair Value Disclosure
Other liabilities	428	419
Fair Value Inputs Level 2 Member \| Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	44,883	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	15,094,299	13,459,334
Fair Value Inputs Level 2 Member \| Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,793,807	3,865,265
Fair Value Inputs Level 2 Member \| Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	892,503	849,802
Fair Value Inputs Level 2 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	482,114	496,626
Fair Value Inputs Level 2 Member \| Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,447,650	1,445,845
Fair Value Inputs Level 2 Member \| US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	125,450	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	465,986	383,365
Fair Value Inputs Level 2 Member \| Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,948,921	1,832,466
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,276,224	2,249,130
Funds withheld at interest embedded derivatives	(3,329)	22,094
Cash and cash equivalents	0	0
Short-term investments	0	0
Investments, Fair Value Disclosure	12,388	7,904
Other Assets, Fair Value Disclosure	10,853	7,727
Assets, Fair Value Disclosure, Recurring	2,296,136	2,286,855
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	1,069,154	1,085,166
Liabilities Fair Value Disclosure
Other liabilities		1,085,963
Liabilities, Fair Value Disclosure, Recurring	1,070,908
Fair Value Inputs Level 3 Member \| Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	12,388	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
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	1,754	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	1,237,317	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	298,376	188,094
Fair Value Inputs Level 3 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	580,510	572,960
Fair Value Inputs Level 3 Member \| Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	77,819	86,746
Fair Value Inputs Level 3 Member \| US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	27,359	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	40,186	42,711
Fair Value Inputs Level 3 Member \| Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 14,657	$ 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