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Summary of Significant Accounting Policies - Recent Accounting Guidance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2017
Recent Accounting Guidance
Tax deductions in excess of recognized compensation cost	$ 29,500
Cash, cash equivalents and restricted cash balances	7,473	$ 6,864
Effect of Change
Recent Accounting Guidance
Increase to retained earnings	1,600
ASU No. 2016-09
Recent Accounting Guidance
Tax deductions in excess of recognized compensation cost	29,500
Excess tax benefits of deferred tax assets	10,300
Reduction in net cash provided by financing activities	$ 1,200	$ 11	$ 8
ASU No. 2016-18
Recent Accounting Guidance
Cash, cash equivalents and restricted cash balances				$ 6,900