Other Assets Other Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Current Assets [Line Items]
Goodwill	$ 330,716	$ 331,884	$ 0
Investments	42,483	41,636
Other Prepaid Expense, Current	20,218	30,332
Derivative Asset	22,447	14,515
Furniture and Fixtures, Gross	6,891	6,123
Deferred Costs and Other Assets	8,426	2,637
Parent Company [Member]
Other Current Assets [Line Items]
Other Assets	$ 431,181	$ 427,127