Revenue from Contracts with Customers Revenue from Contracts with Customers (Balance Sheet Impact of the Adoption of ASC606) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS
Prepaid expenses and other	$ 8,731		$ 6,233
Other assets	28,113	$ 27,028	16,230
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	14,036	15,750	13,002
Other long-term liabilities	102,928	109,940	100,203
Deferred income taxes	158,232	133,064	133,477
Retained earnings	811,781	798,128	$ 799,402
Mid-Stream [Member] | Adjustments due to ASC606 [Member]
ASSETS
Prepaid expenses and other	128
Other assets	11,887	10,798
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	2,875	2,748
Other long-term liabilities	8,456	9,737
Deferred income taxes	168	(413)
Retained earnings	516
Impact of Restatement on Opening Retained Earnings, Net of Tax		$ (1,274)
Mid-Stream [Member] | Amounts without the Adoption of ASC606 [Member]
ASSETS
Prepaid expenses and other	8,603
Other assets	16,226
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of other long-term liabilities	11,161
Other long-term liabilities	94,472
Deferred income taxes	158,064
Retained earnings	$ 811,265