Schedule I - Financial Information of the Registrant (Parent Company) - Statements of Income and Retained Earnings, Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Condensed Consolidating Statement of Operations [Line Items]
Other operating expenses	$ 345,307	$ 351,511	$ 363,986
Affiliates
Schedule of Condensed Consolidating Statement of Operations [Line Items]
Other operating expenses	$ 276	$ 276	$ 276