Investments (Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Dec. 31, 2020 USD ($)
Available-for-sale, Amortized Cost
Due in one year or less	$ 5,472
Due after one year through five years	59,769
Due after five years through ten years	248,035
Due after ten years	765,198
Total	1,078,474
Available-for-sale, Fair Value
Due in one year or less	5,495
Due after one year through five years	61,338
Due after five years through ten years	263,889
Due after ten years	785,091
Total	$ 1,115,813