Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined pension plan benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost – benefits earned during the year	$ 7	$ 27	$ 20	$ 82
Interest cost on projected benefit obligations	149	453	936	1,358
Expected return on plan assets	(399)	(544)	(1,197)	(1,630)
Amortization of prior service cost	0	0	0	0
Amortization of net loss (benefit)	143	156	419	466
Settlement expense	0	740	0	740
Net periodic benefit cost	(100)	832	178	1,016
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost – benefits earned during the year	28	34	91	103
Interest cost on projected benefit obligations	29	64	146	193
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(6)	(6)	(19)	(19)
Amortization of net loss (benefit)	(12)	0	(36)	0
Settlement expense	0	0	0	0
Net periodic benefit cost	$ 39	$ 92	$ 182	$ 277