Basis of Financial Statement Presentation - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment to retained earnings					$ 0
Selling, general and administrative expenses	$ (39,668,000)	$ (34,372,000)	$ (78,564,000)	$ (68,285,000)
Other income (expense)	92,000	815,000	226,000	1,173,000
Accounting Standards Update 2017-07 | Reclassification
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses	62,000	150,000	124,000	300,000
Other income (expense)	$ 62,000	$ 150,000	$ 124,000	$ 300,000