Supplementary Balance Sheet Information - Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Supplementary Balance Sheet Information
Accrued third-party commissions	$ 43,468	$ 34,276
Accrued expenses	38,993	38,018
Dividends payable	24,351	24,886
Accrued interest	13,422	19,330
Litigation settlements	2,691	1,380
Income taxes payable	1,331	185
Other	118,989	107,847
Total	$ 243,245	$ 225,922