Retirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Pension contributions	$ 0	$ 0
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,897	2,940
Interest cost	2,361	2,457
Expected return on plan assets	(2,484)	(2,903)
Amortization of prior service cost		5
Amortization of net (income) loss	2,014	2,251
Net periodic benefit cost	4,788	4,750
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	438	401
Interest cost	296	263
Amortization of net (income) loss	48	211
Net periodic benefit cost	$ 782	$ 875