Basis of Financial Statement Presentation - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Selling, general and administrative expenses	$ (38,523,000)	$ (37,178,000)	$ (117,087,000)	$ (105,463,000)
Other income (expense)	265,000	1,561,000	491,000	2,734,000
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	187,000	450,000
Other income (expense)	$ 62,000	$ 150,000	$ 187,000	$ 450,000
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment to retained earnings					$ 0