Supplemental Balance Sheet, Income Statement and Cash Flow Information - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued liabilities
Accrued compensation	$ 130,970	$ 116,775
Deferred revenue	218,370	255,626
Other taxes payable	32,095	16,419
Workers' compensation liabilities	13,987	18,255
Interest payable	65,642	57,233
Litigation reserves	18,830	24,896
Current liability to discontinued operations	6,074	5,462
Dividends declared and payable	17,148	17,039
Other accrued liabilities	29,928	31,543
Accrued liabilities	$ 533,044	$ 543,248