Schedule I - Financial Information of the Registrant (Parent Company) - Statements of Operations and Retained Earnings (Additional Information) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Condensed Consolidating Statement of Operations [Line Items]
Other operating expenses	$ 351,511	$ 363,986	$ 381,954
Affiliates
Schedule of Condensed Consolidating Statement of Operations [Line Items]
Other operating expenses	$ 276	$ 276	$ 276