Postretirement Benefits (Details 7) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amounts included in accumulated other comprehensive income
Net actuarial loss	$ 191,264	$ 185,278
Prior service cost	10,201	6,023
Net amount not yet recognized	201,465	$ 191,301
Amounts that will be amortized from accumulated other comprehensive income into pension cost during next fiscal year
Estimated net actuarial loss that will be amortized from accumulated other comprehensive income into pension cost during next fiscal year, before tax	12,800
Estimated net prior service cost that will be amortized from accumulated other comprehensive income into pension cost during next fiscal year, before tax	$ 1,100