Investments - Investments Classified by Contractual Maturity Date (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 2,257
Due after one year through five years	113,163
Due after five years through ten years	40,811
Due after ten years	4,306
Mortgage-backed securities	429,351
Total	589,888	587,555
Estimated Fair Value
Due in one year or less	2,284
Due after one year through five years	115,615
Due after five years through ten years	42,347
Due after ten years	4,134
Mortgage-backed securities	435,356
Total	$ 599,736	$ 593,182