Pension Benefits and Other Postretirement Benefit Plans (Additional Information) (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) plan	Jun. 30, 2017 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of qualified noncontributory defined benefit plans | plan		2
Anticipated payment for defined benefit pensions		$ 338	$ 147
Bear Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Required employer contribution	$ 100