Revenue (Impact on Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other assets	$ 35,074		$ 8,291
Total assets	1,678,587		1,651,983
Retained earnings	103,764		72,741
Total stockholders' equity	659,896		$ 612,250
ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total assets		$ 9,300
Retained earnings		$ 6,900
Balances without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other assets	27,970
Total assets	1,671,483
Retained earnings	96,660
Total stockholders' equity	652,792
Adjustments | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other assets	(7,104)
Total assets	(7,104)
Retained earnings	(7,104)
Total stockholders' equity	$ (7,104)