Fair Value Measurements and Derivative Instruments (Reconciliation) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Amortized cost
Within 1 year	$ 38,624
After 1 year through 5 years	111,756
After 5 years through 10 years	68,599
After 10 years	146,797
Total	365,776
Estimated fair value
Within 1 year	38,414
After 1 year through 5 years	111,704
After 5 years through 10 years	69,154
After 10 years	148,561
Total	$ 367,833