Investments - Investments Classified by Contractual Maturity Date (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 1,594
Due after one year through five years	145,156
Due after five years through ten years	28,008
Due after ten years	6,426
Mortgage-backed securities	292,739
Total	473,923	627,462
Estimated Fair Value
Due in one year or less	1,608
Due after one year through five years	148,476
Due after five years through ten years	29,210
Due after ten years	6,160
Mortgage-backed securities	297,553
Total	$ 483,007	$ 640,212