Fair Value Measurements and Derivative Instruments (Available-for-sale Securities Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized cost
Within 1 year	$ 54,475
After 1 year through 5 years	74,200
After 5 years through 10 years	37,291
After 10 years	72,767
Total	238,733	$ 270,313
Estimated fair value
Within 1 year	54,348
After 1 year through 5 years	73,898
After 5 years through 10 years	37,393
After 10 years	71,733
Total	$ 237,372	$ 265,329