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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenues	$ 66,900	$ 48,100	$ 67,300	$ 56,900	$ 101,600	$ 76,600	$ 90,500	$ 66,900	$ 239,182	$ 335,519	$ 261,560
Income (loss) before taxes	(1,700)	$ (10,300)	$ 9,600	$ 2,100	32,400	$ 19,200	$ 29,900	$ 6,500	(298)	87,881	43,295
Total assets	610,817				456,682				610,817	456,682	423,144
North America
Segment Reporting Information [Line Items]
Total revenues									169,502	206,673	172,582
Income (loss) before taxes									15,315	48,927	40,895
Total assets	426,799				266,975				426,799	266,975	261,077
Europe
Segment Reporting Information [Line Items]
Total revenues									47,441	110,229	63,875
Income (loss) before taxes									(10,547)	52,650	9,577
Total assets	48,195				64,467				48,195	64,467	38,409
Rest of World
Segment Reporting Information [Line Items]
Total revenues									22,239	18,617	25,103
Income (loss) before taxes									(5,066)	(13,696)	(7,177)
Total assets	$ 135,823				$ 125,240				$ 135,823	$ 125,240	$ 123,658