Revenue from Contracts with Customers (Balance Sheet Impact of the Adoption of ASC606) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS
Prepaid expenses and other	$ 11,356		$ 6,172
Other assets	27,816	$ 27,028	16,230
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	14,250	15,750	13,002
Other long-term liabilities	101,234	109,940	100,203
Deferred income taxes	144,748	133,064	133,477
Retained earnings	752,840	798,128	$ 799,402
Mid-Stream | Adjustments due to ASC606 [Member]
ASSETS
Prepaid expenses and other	285
Other assets	12,879	10,798
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	2,874	2,748
Other long-term liabilities	7,007	9,737
Deferred income taxes	805	(413)
Retained earnings	2,478	$ (1,274)
Mid-Stream | Amounts without the adoption of ASC606 [Member]
ASSETS
Prepaid expenses and other	11,071
Other assets	14,937
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	11,376
Other long-term liabilities	94,227
Deferred income taxes	143,943
Retained earnings	$ 750,362