Investments - Investments Classified by Contractual Maturity Date (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 2,701
Due after one year through five years	161,850
Due after five years through ten years	29,087
Due after ten years	6,603
Mortgage-backed securities	427,221
Total	627,462	587,555
Estimated Fair Value
Due in one year or less	2,728
Due after one year through five years	165,719
Due after five years through ten years	30,585
Due after ten years	6,339
Mortgage-backed securities	434,841
Total	$ 640,212	$ 593,182