Income Taxes - Income Tax Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations		$ 2,235	$ 3,071	$ 19,465
Accrued interest and penalties related to unrecognized tax benefits		300	200
Interest and penalties recorded		100	(400)	(800)
Forecast | Subsequent event
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations	$ 2,600
Additional Paid in Capital
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations				2,600
Deferred tax assets
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations		700	1,000
Deferred tax assets | Forecast | Subsequent event
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations	800
Income Tax Expense
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations		1,500	2,000	16,000
Interest expense related to unrecognized tax benefits		$ (100)	(400)	$ (1,400)
Income Tax Expense | Forecast | Subsequent event
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations	$ 1,800
Income Tax Expense | State
Income tax contingency disclosures
Unrecognized tax benefits: Changes due to lapses of statutes of limitations			$ 700