Income Taxes - Reconciliation of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Provision at statutory rate	$ 19,192	$ 17,445	$ 14,403
State income taxes, net of federal tax benefit	1,292	923	1,242
Valuation allowance	564	0	0
Foreign - tax rate differential and other	29	(87)	(13)
Domestic production activities deduction	(721)	(560)	(521)
Federal and state credits	(542)	0	0
Other	448	(172)	103
Provision for income taxes	$ 20,262	$ 17,549	$ 15,214