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Pension and Other Postretirement Benefit Plans - Summary of Net Period Benefit Costs (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Compensation and Retirement Disclosure [Abstract]        
Interest cost $ 4,774 $ 4,886 $ 14,323 $ 14,688
Expected return on plan assets (5,279) (5,909) (15,838) (17,959)
Amortization of prior service credit (358) (358) (1,075) (1,074)
Amortization of actuarial loss, net 1,761 1,257 5,284 3,591
Net periodic cost (credit) $ 898 $ (124) $ 2,694 $ (754)