Investments - Investments Classified by Contractual Maturity Date (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available for sale, Amortized Cost
Due in one year or less	$ 4,331
Due after one year through five years	100,610
Due after five years through ten years	25,767
Due after ten years	2,900
Mortgage-backed securities	270,455
Amortized Cost	404,063	396,782
Available for sale, Estimated Fair Value
Due in one year or less	4,382
Due after one year through five years	102,383
Due after five years through ten years	26,561
Due after ten years	2,620
Mortgage-backed securities	274,115
Available for sale, fair value	410,061	400,146
Held to maturity, Amortized Cost
Due in one year or less	0
Due after one year through five years	662
Due after five years through ten years	12,963
Due after ten years	1,258
Mortgage-backed securities	30,680
Amortized Cost	45,563	45,985
Held to maturity, Estimated Fair Value
Due in one year or less	0
Due after one year through five years	664
Due after five years through ten years	12,875
Due after ten years	1,237
Mortgage-backed securities	31,035
Held to maturity, fair value	$ 45,811	$ 45,795