Investments (Details 1) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Summary of amortized cost and estimated fair value of investments excluding VIE investments by contractual maturity
Amortized cost due in one year or less	$ 806,147
Estimated fair value due in one year or less	809,446
Amortized cost due after one year through five years	964,876
Estimated fair value due after one year through five years	1,010,276
Amortized cost due after five years through ten years	1,139,323
Estimated fair value due after five years through ten years	1,235,519
Amortized cost due after ten years	1,448,763
Estimated fair value due after ten years	1,543,965
Amortized Cost	4,359,109
Estimated Fair Value	4,599,206
Amortized cost of investments	6,242,004	6,391,970
Estimated Fair Value	6,687,109	6,876,990
Residential Mortgage-backed Securities [Member]
Summary of amortized cost and estimated fair value of investments excluding VIE investments by contractual maturity
Amortized Cost	1,065,898
Estimated Fair Value	1,275,346
Collateralized debt obligations [Member]
Summary of amortized cost and estimated fair value of investments excluding VIE investments by contractual maturity
Amortized Cost	42,099
Estimated Fair Value	41,366
Other asset-backed securities [Member]
Summary of amortized cost and estimated fair value of investments excluding VIE investments by contractual maturity
Amortized Cost	774,898
Estimated Fair Value	$ 771,191