Postretirement Benefits (Details 7) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Amounts included in accumulated other comprehensive income
Net actuarial loss	$ 185,278	$ 148,865
Prior service cost	6,023	4,605
Net amount not yet recognized	191,301	$ 153,470
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	9,300
Estimated net prior service cost that will be amortized from accumulated other comprehensive income into pension cost during next fiscal year, before tax	$ 800