Subsequent Event (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended
	Jul. 31, 2012
Amortized Cost Recorded Investment [Member] \| Corporate Debt Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	$ 2,585,095
Amortized Cost Recorded Investment [Member] \| Asset Backed Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	137,251
Amortized Cost Recorded Investment [Member] \| Commercial Mortgage Backed Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	703,313
Amortized Cost Recorded Investment [Member] \| US Government Corporations and Agencies Securities [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	240,952
Amortized Cost Recorded Investment [Member] \| US States And Political Subdivisions Member
Subsequent Event [Line Items]
Subsequent Event, Amount	27,297
Amortized Cost Recorded Investment [Member] \| Other Foreign Government, Supranational And Foreign Government-Sponsored Enterprises
Subsequent Event [Line Items]
Subsequent Event, Amount	56,776
Amortized Cost Recorded Investment [Member] \| Mortgage Loans On Real Estate Member
Subsequent Event [Line Items]
Subsequent Event, Amount	1,009,454
Amortized Cost Recorded Investment [Member] \| Short Term Investments Member
Subsequent Event [Line Items]
Subsequent Event, Amount	101,428
Amortized Cost Recorded Investment [Member] \| Cash and Cash Equivalents [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	501,593
Amortized Cost Recorded Investment [Member] \| Accrued Income Receivable [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	43,739
Amortized Cost Recorded Investment [Member] \| Availableforsale Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	3,750,684
Amortized Cost Recorded Investment [Member] \| Total Subsequent Event [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	5,406,898
Estimate of Fair Value, Fair Value Disclosure [Member] \| Corporate Debt Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	2,606,816
Estimate of Fair Value, Fair Value Disclosure [Member] \| Asset Backed Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	138,918
Estimate of Fair Value, Fair Value Disclosure [Member] \| Commercial Mortgage Backed Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	704,065
Estimate of Fair Value, Fair Value Disclosure [Member] \| US Government Corporations and Agencies Securities [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	256,168
Estimate of Fair Value, Fair Value Disclosure [Member] \| US States And Political Subdivisions Member
Subsequent Event [Line Items]
Subsequent Event, Amount	27,555
Estimate of Fair Value, Fair Value Disclosure [Member] \| Other Foreign Government, Supranational And Foreign Government-Sponsored Enterprises
Subsequent Event [Line Items]
Subsequent Event, Amount	55,437
Estimate of Fair Value, Fair Value Disclosure [Member] \| Mortgage Loans On Real Estate Member
Subsequent Event [Line Items]
Subsequent Event, Amount	1,021,661
Estimate of Fair Value, Fair Value Disclosure [Member] \| Short Term Investments Member
Subsequent Event [Line Items]
Subsequent Event, Amount	101,338
Estimate of Fair Value, Fair Value Disclosure [Member] \| Cash and Cash Equivalents [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	501,593
Estimate of Fair Value, Fair Value Disclosure [Member] \| Accrued Income Receivable [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	43,739
Estimate of Fair Value, Fair Value Disclosure [Member] \| Availableforsale Securities Member
Subsequent Event [Line Items]
Subsequent Event, Amount	3,788,959
Estimate of Fair Value, Fair Value Disclosure [Member] \| Total Subsequent Event [Member]
Subsequent Event [Line Items]
Subsequent Event, Amount	$ 5,457,290