Investments (Details 5) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Cash and securities pledged directly from the investment portfolio [Member]
Schedule of sources of collateral received or held and how that collateral was pledged
Fair Value of Cash and Underlying Securities	$ 776,523	$ 874,987
Fair Value of Cash and Securities Pledged to Investment and Repurchase Agreement Counterparties	461,408	564,036
Fair Value of Cash and Securities Pledged to Derivative Counterparties	315,115	310,951
Cash and securities pledged from its derivative counterparties [Member]
Schedule of sources of collateral received or held and how that collateral was pledged
Fair Value of Cash and Underlying Securities
Fair Value of Cash and Securities Pledged to Investment and Repurchase Agreement Counterparties
Fair Value of Cash and Securities Pledged to Derivative Counterparties