Income Taxes - Allocation of Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax (benefit) expense	$ 3,531	$ 1,828	$ 5,359	$ 15,517	$ (53,133)	$ 2,840
Other comprehensive income for changes in cash flow hedge				16	522	4,162
Income tax expense				$ 15,533	$ (52,611)	$ 7,002