Fair Value of Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 21,284,216	$ 22,291,614
Funds withheld at interest embedded derivatives	(108,473)	(243,177)
Cash and cash equivalents	347,823	575,864
Short-term investments	21,092	239,131
Other invested assets	293,889	419,861
Total Assets	21,838,547	23,283,293
Liabilities Fair Value Disclosure
Interest-sensitive contract liabilities - embedded derivatives	878,568	912,361
Total Liabilities	891,687	944,013
Non-redeemable preferred stock
Assets Fair Value Disclosure
Other invested assets	90,458	74,841
Other equity securities
Assets Fair Value Disclosure
Other invested assets	69,881	147,859
Securities Pledged As Collateral Member
Assets Fair Value Disclosure
Other invested assets		17,002
Other
Assets Fair Value Disclosure
Other invested assets	14,759	11,951
Interest Rate Swaps
Assets Fair Value Disclosure
Other invested assets	36,853	104,972
Liabilities Fair Value Disclosure
Other liabilities	4,140	196
Interest Rate Options
Assets Fair Value Disclosure
Other invested assets	8,073
Foreign Currency Forwards
Assets Fair Value Disclosure
Other invested assets	148	1,017
Liabilities Fair Value Disclosure
Other liabilities	9,484	2,105
Equity Option
Assets Fair Value Disclosure
Other invested assets	60,715	62,514
Liabilities Fair Value Disclosure
Other liabilities	(387)
Consumer Price Index Swaps
Assets Fair Value Disclosure
Other invested assets	(23)	1,446
Credit Default Swaps
Assets Fair Value Disclosure
Other invested assets	(2,775)	(1,741)
Liabilities Fair Value Disclosure
Other liabilities	(118)	1,953
Foreign Currency Swaps
Assets Fair Value Disclosure
Other invested assets	15,800
Liabilities Fair Value Disclosure
Other liabilities		27,398
Corporate securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	11,803,481	12,380,071
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,525,793	4,049,334
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,054,968	1,042,064
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	790,310	691,555
Commercial mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,638,632	1,698,903
US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	412,990	265,190
State and political subdivisions
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	291,811	302,498
Other foreign government, supranational and foreign government-sponsored enterprises
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,766,231	1,861,999
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	708,373	533,349
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	347,823	575,864
Short-term investments	19,169	178,923
Other invested assets	172,368	225,401
Total Assets	1,247,733	1,513,537
Liabilities Fair Value Disclosure
Interest-sensitive contract liabilities - embedded derivatives	0	0
Total Liabilities	0	0
Fair Value Inputs Level 1 Member \| Non-redeemable preferred stock
Assets Fair Value Disclosure
Other invested assets	87,728	64,268
Fair Value Inputs Level 1 Member \| Other equity securities
Assets Fair Value Disclosure
Other invested assets	69,881	147,859
Fair Value Inputs Level 1 Member \| Securities Pledged As Collateral Member
Assets Fair Value Disclosure
Other invested assets		1,323
Fair Value Inputs Level 1 Member \| Other
Assets Fair Value Disclosure
Other invested assets	14,759	11,951
Fair Value Inputs Level 1 Member \| Interest Rate Swaps
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member \| Interest Rate Options
Assets Fair Value Disclosure
Other invested assets	0
Fair Value Inputs Level 1 Member \| Foreign Currency Forwards
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member \| Equity Option
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0
Fair Value Inputs Level 1 Member \| Consumer Price Index Swaps
Assets Fair Value Disclosure
Other invested assets	0	0
Fair Value Inputs Level 1 Member \| Credit Default Swaps
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member \| Foreign Currency Swaps
Assets Fair Value Disclosure
Other invested assets	0
Liabilities Fair Value Disclosure
Other liabilities		0
Fair Value Inputs Level 1 Member \| Corporate securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	63,568	43,544
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
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	354,097	192,780
Fair Value Inputs Level 1 Member \| State and political subdivisions
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member \| Other foreign government, supranational and foreign government-sponsored enterprises
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	290,708	297,025
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	18,242,091	19,520,344
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	0	0
Short-term investments	1,923	38,177
Other invested assets	121,521	194,460
Total Assets	18,365,535	19,752,981
Liabilities Fair Value Disclosure
Interest-sensitive contract liabilities - embedded derivatives	0	0
Total Liabilities	13,119	31,652
Fair Value Inputs Level 2 Member \| Non-redeemable preferred stock
Assets Fair Value Disclosure
Other invested assets	2,730	10,573
Fair Value Inputs Level 2 Member \| Other equity securities
Assets Fair Value Disclosure
Other invested assets	0	0
Fair Value Inputs Level 2 Member \| Securities Pledged As Collateral Member
Assets Fair Value Disclosure
Other invested assets		15,679
Fair Value Inputs Level 2 Member \| Other
Assets Fair Value Disclosure
Other invested assets	0	0
Fair Value Inputs Level 2 Member \| Interest Rate Swaps
Assets Fair Value Disclosure
Other invested assets	36,853	104,972
Liabilities Fair Value Disclosure
Other liabilities	4,140	196
Fair Value Inputs Level 2 Member \| Interest Rate Options
Assets Fair Value Disclosure
Other invested assets	8,073
Fair Value Inputs Level 2 Member \| Foreign Currency Forwards
Assets Fair Value Disclosure
Other invested assets	148	1,017
Liabilities Fair Value Disclosure
Other liabilities	9,484	2,105
Fair Value Inputs Level 2 Member \| Equity Option
Assets Fair Value Disclosure
Other invested assets	60,715	62,514
Liabilities Fair Value Disclosure
Other liabilities	(387)
Fair Value Inputs Level 2 Member \| Consumer Price Index Swaps
Assets Fair Value Disclosure
Other invested assets	(23)	1,446
Fair Value Inputs Level 2 Member \| Credit Default Swaps
Assets Fair Value Disclosure
Other invested assets	(2,775)	(1,741)
Liabilities Fair Value Disclosure
Other liabilities	(118)	1,953
Fair Value Inputs Level 2 Member \| Foreign Currency Swaps
Assets Fair Value Disclosure
Other invested assets	15,800
Liabilities Fair Value Disclosure
Other liabilities		27,398
Fair Value Inputs Level 2 Member \| Corporate securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	10,147,908	10,667,964
Fair Value Inputs Level 2 Member \| Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,525,793	4,049,334
Fair Value Inputs Level 2 Member \| Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	914,914	948,133
Fair Value Inputs Level 2 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	437,704	459,164
Fair Value Inputs Level 2 Member \| Commercial mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,457,650	1,531,897
Fair Value Inputs Level 2 Member \| US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	58,893	67,872
Fair Value Inputs Level 2 Member \| State and political subdivisions
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	250,536	259,286
Fair Value Inputs Level 2 Member \| Other foreign government, supranational and foreign government-sponsored enterprises
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,448,693	1,536,694
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,333,752	2,237,921
Funds withheld at interest embedded derivatives	(108,473)	(243,177)
Cash and cash equivalents	0	0
Short-term investments	0	22,031
Other invested assets	0	0
Total Assets	2,225,279	2,016,775
Liabilities Fair Value Disclosure
Interest-sensitive contract liabilities - embedded derivatives	878,568	912,361
Total Liabilities	878,568	912,361
Fair Value Inputs Level 3 Member \| Non-redeemable preferred stock
Assets Fair Value Disclosure
Other invested assets	0	0
Fair Value Inputs Level 3 Member \| Other equity securities
Assets Fair Value Disclosure
Other invested assets	0	0
Fair Value Inputs Level 3 Member \| Securities Pledged As Collateral Member
Assets Fair Value Disclosure
Other invested assets		0
Fair Value Inputs Level 3 Member \| Other
Assets Fair Value Disclosure
Other invested assets	0	0
Fair Value Inputs Level 3 Member \| Interest Rate Swaps
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member \| Interest Rate Options
Assets Fair Value Disclosure
Other invested assets	0
Fair Value Inputs Level 3 Member \| Foreign Currency Forwards
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member \| Equity Option
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0
Fair Value Inputs Level 3 Member \| Consumer Price Index Swaps
Assets Fair Value Disclosure
Other invested assets	0	0
Fair Value Inputs Level 3 Member \| Credit Default Swaps
Assets Fair Value Disclosure
Other invested assets	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member \| Foreign Currency Swaps
Assets Fair Value Disclosure
Other invested assets	0
Liabilities Fair Value Disclosure
Other liabilities		0
Fair Value Inputs Level 3 Member \| Corporate securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,592,005	1,668,563
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	140,054	93,931
Fair Value Inputs Level 3 Member \| Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	352,606	232,391
Fair Value Inputs Level 3 Member \| Commercial mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	180,982	167,006
Fair Value Inputs Level 3 Member \| US Treasury And Government Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	4,538
Fair Value Inputs Level 3 Member \| State and political subdivisions
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	41,275	43,212
Fair Value Inputs Level 3 Member \| Other foreign government, supranational and foreign government-sponsored enterprises
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 26,830	$ 28,280