Merger, Integration and Other Costs (Summary of Merger and Restructuring Costs) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Merger Integration And Other Costs [Abstract]
Merger costs		$ 892		$ 11,515
Restructuring costs:
Termination benefits				8,782
Professional services		23		678
Other		57		270
Total restructuring costs		80		9,730
Total merger and restructuring costs	[1]	$ 972	$ 27	$ 21,245	$ 27

[1] For integration and restructuring costs related to the merger with Deltic see Note 13: Merger, Integration and Other Costs.