Goodwill (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Changes in the carrying amount of goodwill
Balance at beginning of year	$ 200,584	$ 149,208	$ 149,208
Additions (See Note 4)	4,578		52,481
Impairment adjustment		$ (1,105)	(1,105)
Balance at end of period	$ 205,162		$ 200,584