Business Information - Information by Geographic Region, After Elimination of All Significant Inter-company Accounts and Transactions (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenues	$ 89,100	$ 86,800	$ 88,500	$ 87,500	$ 66,900	$ 48,100	$ 67,300	$ 56,900	$ 351,985	$ 239,182	$ 335,519
Operating income (loss)	21,600	$ 20,700	$ 19,500	$ 19,000	3,000	$ (9,400)	$ 10,400	$ 2,900	80,868	6,900	91,108
Total assets	485,700				610,817				485,700	610,817	456,682
North America
Segment Reporting Information [Line Items]
Total revenues									193,707	169,502	206,673
Operating income (loss)									16,472	19,887	48,788
Total assets	198,313				426,799				198,313	426,799	266,975
Europe
Segment Reporting Information [Line Items]
Total revenues									125,149	47,441	110,229
Operating income (loss)									57,736	(10,541)	52,659
Total assets	152,478				48,195				152,478	48,195	64,467
Rest of World
Segment Reporting Information [Line Items]
Total revenues									33,129	22,239	18,617
Operating income (loss)									6,660	(2,446)	(10,339)
Total assets	$ 134,909				$ 135,823				$ 134,909	$ 135,823	$ 125,240