Benefit Plans - Summary of Components of Net Periodic (Benefit) Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Benefit Plans
Service cost	$ 0	$ 0	$ 0
Interest cost	9,929	10,450	9,549
Expected return on plan assets	(14,534)	(14,843)	(12,754)
Recognized net actuarial loss (gain)	2,973	1,401	3,138
Amortization of prior service cost	190	190	213
Net periodic (benefit) cost	(1,442)	(2,802)	146
Other Postretirement Benefits
Benefit Plans
Service cost	2,129	1,241	1,578
Interest cost	1,004	930	1,279
Expected return on plan assets	0	0	0
Recognized net actuarial loss (gain)	0	(488)	472
Amortization of prior service cost	0	0	0
Net periodic (benefit) cost	$ 3,133	$ 1,683	$ 3,329