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Pension and Other Postretirement Benefit Plans - Summary of Net Period Benefit Costs (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Compensation and Retirement Disclosure [Abstract]        
Interest cost $ 4,799 $ 4,900 $ 9,549 $ 9,802
Expected return on plan assets (5,259) (6,025) (10,559) (12,050)
Amortization of prior service credit (359) (358) (717) (716)
Amortization of actuarial loss, net 1,798 1,167 3,523 2,334
Net periodic cost (credit) $ 979 $ (316) $ 1,796 $ (630)