Other Assets Other Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Current Assets [Line Items]
Goodwill	$ 318,710	$ 331,884	$ 0
Investments	44,014	41,636
Other Prepaid Expense, Current	24,093	30,332
Derivative Asset	26,802	14,515
Furniture and Fixtures, Gross	6,715	6,123
Deferred Costs and Other Assets	7,392	2,637
Parent Company [Member]
Other Current Assets [Line Items]
Other Assets	$ 427,726	$ 427,127