Benefit Plans - Summary of Components of Net Periodic (Benefit) Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Benefit Plans
Service cost	$ 0	$ 0	$ 0
Interest cost	10,042	9,929	10,450
Expected return on plan assets	(12,232)	(14,534)	(14,843)
Recognized net actuarial loss (gain)	2,813	2,973	1,401
Amortization of prior service cost	190	190	190
Net periodic (benefit) cost	813	(1,442)	(2,802)
Other Postretirement Benefits
Benefit Plans
Service cost	1,863	2,129	1,241
Interest cost	994	1,004	930
Expected return on plan assets	0	0	0
Recognized net actuarial loss (gain)	(423)	0	(488)
Amortization of prior service cost	0	0	0
Net periodic (benefit) cost	$ 2,434	$ 3,133	$ 1,683