Supplementary Balance Sheet Information - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Supplementary Balance Sheet Information
Prepaid expenses	$ 59,275	$ 65,159
Income taxes receivable	29,834	41,400
R and D state tax credit	22,642	22,642
Supplies inventory	19,261	17,072
Other	71,033	70,292
Total	$ 202,045	$ 216,565