Fair Value Measurements - Assets and liabilities at fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liabilities
Fair value asset, Transfer between Level 1 to Level 2	$ 0	$ 0
Fair value asset, Transfer between Level 2 to Level 1	0	0
Fair value Liabilities, Transfer between Level 1 to Level 2	0	0
Fair value Liabilities, Transfer between Level 2 to Level 1	0	0
Level 1
Assets
Total assets	12,503	14,088
Liabilities
Total liabilities	1,866	1,846
Level 2
Assets
Total assets	5,237	16,643
Liabilities
Total liabilities	6,636	1,138
Other current assets | Rabbi trust | Level 1
Assets
Total assets	10,637	6,994
Other current assets | Foreign exchange derivative instruments | Level 2
Assets
Total assets	4,675	11,357
Other noncurrent assets | Rabbi trust | Level 1
Assets
Total assets		5,248
Other noncurrent assets | Foreign exchange derivative instruments | Level 2
Assets
Total assets	562	5,286
Other noncurrent assets | Deferred compensation program assets | Level 1
Assets
Total assets	1,866	1,846
Other current liabilities | Foreign exchange derivative instruments | Level 2
Liabilities
Total liabilities	6,360	1,106
Other noncurrent liabilities | Foreign exchange derivative instruments | Level 2
Liabilities
Total liabilities	276	32
Other noncurrent liabilities | Deferred compensation program liabilities | Level 1
Liabilities
Total liabilities	$ 1,866	$ 1,846