Fair Value Measurements and Derivative Instruments (Available-for-sale Securities Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized cost
Within 1 year	$ 54,573
After 1 year through 5 years	111,948
After 5 years through 10 years	59,377
After 10 years	145,930
Total	371,828	$ 365,776
Estimated fair value
Within 1 year	54,230
After 1 year through 5 years	110,591
After 5 years through 10 years	57,862
After 10 years	142,395
Total	$ 365,078	$ 367,833