Income Taxes - Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
U.S. federal	$ 14,409	$ 6,765	$ 858
U.S. state	1,887	318	171
Non-U.S.	330	118	355
Total current	16,626	7,201	1,384
Deferred:
U.S. federal	(9,418)	8,130	11,324
U.S. state	819	1,037	573
Non-U.S.	53	112	(62)
Total deferred	(8,546)	9,279	11,835
Total income tax expense (benefit)	$ 8,080	$ 16,480	$ 13,219