Fair Value Measurements and Derivative Instruments (Available-for-sale Securities Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized cost
Within 1 year	$ 13,147
After 1 year through 5 years	21,318
After 5 years through 10 years	60,188
After 10 years	237,605
Amortized cost	332,258	$ 325,421
Estimated fair value
Within 1 year	13,149
After 1 year through 5 years	21,261
After 5 years through 10 years	58,612
After 10 years	233,404
Total	$ 326,426	$ 325,794