Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined pension plan benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost – benefits earned during the year	$ 35		$ 7	$ 105	$ 20
Interest cost on projected benefit obligations	6		149	323	936
Expected return on plan assets	0		(399)	(835)	(1,197)
Amortization of prior service cost	0		0	0	0
Amortization of net loss (benefit)	13		143	302	419
Settlement expense	11,732	$ 8,200	0	19,900	0
Net periodic benefit cost	11,786		(100)	19,795	178
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost – benefits earned during the year	31		28	93	91
Interest cost on projected benefit obligations	47		29	142	146
Expected return on plan assets	0		0	0	0
Amortization of prior service cost	(3)		(6)	(10)	(19)
Amortization of net loss (benefit)	(8)		(12)	(23)	(36)
Settlement expense	0		0	0	0
Net periodic benefit cost	$ 67		$ 39	$ 202	$ 182