Defined Benefit and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net periodic benefit cost [Abstract]
Recognized net loss due to defined benefit plan termination	$ 17,644		$ 17,644
Supplemental Retirement Plan [Member]
Net periodic benefit cost [Abstract]
Service cost	35	$ 74	70	$ 148
Interest cost	514	542	1,075	1,088
Amortization of prior service cost	62	62	125	125
Recognized net actuarial loss	215	214	437	435
Net periodic benefit cost	$ 826	$ 892	$ 1,707	$ 1,796