Employee Benefit Plans - Change in Postretirement Benefit Obligation and Plan Assets (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Change in Benefit Obligation
Benefit obligation at beginning of the period	$ 37,482	$ 36,626	$ 36,626
Service cost	1,163	1,743	2,323
Interest cost	810	$ 1,123	1,498
Actuarial (gain) loss	3,084		(2,846)
Benefits paid	(817)		(934)
Curtailment	(4,185)		0
Plan amendments	(8,643)		815
Benefit obligation at end of the period	28,894		37,482
Change in Plan Assets
Fair value of plan assets at end of the period	0		0
Funded status at end of the period	$ (28,894)		$ (37,482)