Defined Benefit Plan - Net Periodic Benefit Cost (Credit) (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
Amount to be amortized from accumulated other comprehensive income			$ 0		$ 0
Components of net periodic benefit cost (credit):
Service cost	$ 153,000	$ 334,000	460,000	$ 334,000
Interest cost	298,000	299,000	894,000	299,000
Expected return on plan assets	(297,000)	(343,000)	(892,000)	(343,000)
Net periodic benefit cost	$ 154,000	$ 290,000	$ 462,000	$ 290,000