Investments - Amortized Cost and Estimated Fair Value of AFS Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized cost
Due in one year or less	$ 263,957
Due after one year through five years	163,276
Due after five years through ten years	217,659
Due after ten years	112,503
Amortized cost	757,395	$ 741,799
Estimated fair value
Due in one year or less	263,837
Due after one year through five years	160,504
Due after five years through ten years	212,692
Due after ten years	111,490
Estimated fair value	$ 748,523	$ 745,961