Income Taxes Unrecognized tax benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Abstract]
(Reductions) additions based on tax positions related to current year		$ 773	$ 1,508
(Reductions) additions based on tax positions related to current year	$ (2,280)
Additions for tax positions of prior years	6,688	8,396	20,684
Reductions for tax positions of prior years	(368)	(2,246)	(8,354)
Settlements	(444)	(364)	(50)
Unrecognized tax benefit balance	37,319	$ 33,723	$ 27,164	$ 13,376
Tax benefits, if recognized, affect annual effective tax rate	6,200
Unrecognized tax benefits, potential reduction	2,400
Accrued for payment of interest related to unrecognized tax benefits	3,300
Interest expense related to unrecognized tax benefits	$ 1,000