Pension and Other Postretirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Contribution to pension and other postemployment benefit plans			$ 13,000
Future contributions to pension and other postemployment benefit plans			16,900
Defined Benefit Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,760	$ 4,235	7,541	$ 8,424
Interest cost	6,466	7,948	12,968	15,808
Expected return on plan assets	(9,860)	(10,530)	(19,775)	(20,944)
Amortization of net loss	5,102	6,378	10,210	12,689
Amortization of prior service credit	(1,238)	(1,126)	(2,483)	(2,241)
Total net pension cost and other postretirement benefit cost	4,230	6,905	8,461	13,736
Other Postretirement Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	114	324	228	644
Interest cost	506	703	1,015	1,398
Amortization of net loss	2	0	1	(2)
Amortization of prior service credit	(96)	0	(192)	0
Total net pension cost and other postretirement benefit cost	$ 526	$ 1,027	$ 1,052	$ 2,040