Fair Value Measurements and Derivative Instruments (Available-for-sale Securities Maturities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized cost
Within 1 year	$ 56,012
After 1 year through 5 years	114,973
After 5 years through 10 years	58,066
After 10 years	154,164
Total	383,215	$ 365,776
Estimated fair value
Within 1 year	55,705
After 1 year through 5 years	113,720
After 5 years through 10 years	56,867
After 10 years	151,838
Total	$ 378,130	$ 367,833