Fair Value Measurements and Derivative Instruments (Available-for-sale Securities Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized cost
Within 1 year	$ 51,133
After 1 year through 5 years	115,266
After 5 years through 10 years	66,418
After 10 years	147,657
Total	380,474	$ 365,776
Estimated fair value
Within 1 year	50,855
After 1 year through 5 years	114,330
After 5 years through 10 years	65,540
After 10 years	146,473
Total	$ 377,198	$ 367,833