Summary of Significant Accounting Policies New Accounting Pronouncements or Change in Accounting Principle (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Provision for income taxes	$ 13,904	$ 11,363	$ 15,115
Effective Income Tax Rate Reconciliation, Percent	41.00%	33.40%	35.80%