Employee Benefit Plans - Change in Benefit Obligation and Fair Value (Details) - Postretirement Benefits Other than Pensions - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in Benefit Obligation
Benefit obligation at beginning of year	$ 36,626	$ 37,076	$ 34,995
Service cost	2,323	1,808	1,295
Interest cost	1,498	1,448	1,343
Actuarial (gain) loss	(2,846)	(2,829)	373
Benefits paid	(934)	(877)	(930)
Plan amendments	815	0	0
Benefit obligation at end of year	37,482	36,626	37,076
Change in Plan Assets
Fair value of plan assets at end of year	0	0	0
Funded status at end of year	$ (37,482)	$ (36,626)	$ (37,076)