Significant Accounting Policies Recent Accounting Adjustments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Income Tax Expense (Benefit)		$ (38)	$ 131	$ (60)
ASU 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect on retained earnings	$ 2,000