Income Taxes - Reconciliation of the Beginning and Ending Balances of our Liability for Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized tax benefits
Balance, beginning of year	$ 1,891	$ 4,079	$ 3,417
Additions based on tax positions related to current year	136		150
Audit settlements	(183)	(1,893)
Lapse of statute of limitations	(174)	(122)
Additions for tax positions of prior years	257	370	925
Reductions for tax positions of prior years	(12)	(543)	(413)
Balance, end of year	$ 1,915	$ 1,891	$ 4,079