Investments - Investments Classified by Contractual Maturity Date (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 2,299
Due after one year through five years	118,719
Due after five years through ten years	42,958
Due after ten years	4,465
Mortgage-backed securities	426,992
Total	595,433	587,555
Estimated Fair Value
Due in one year or less	2,325
Due after one year through five years	122,152
Due after five years through ten years	45,014
Due after ten years	4,303
Mortgage-backed securities	436,563
Total	$ 610,357	$ 593,182