Stock Based Compensation Schedule of compensation cost for share-based payment arrangements, allocation of share-based compensation costs by plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation	$ 30,631	$ 16,052	$ 15,513
Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation	24,899	11,049	11,111
PerformanceStock​Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation	1,707	1,305	972
Incentive Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation	$ 0	$ 0	$ 0