Retirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Pension contributions	$ 0	$ 0	$ 0	$ 0
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,203	2,460	9,398	7,381
Interest cost	2,758	2,542	7,926	7,625
Expected return on plan assets	(2,904)	(2,822)	(8,710)	(8,468)
Amortization of prior service cost	4	13	15	38
Amortization of net (income) loss	2,312	1,173	6,824	3,356
FAS 88 settlement charge		5,269		5,269
Net periodic benefit cost	5,373	8,635	15,453	15,201
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	498	407	1,498	1,221
Interest cost	329	342	987	1,026
Amortization of net (income) loss	76	82	497	246
Net periodic benefit cost	$ 903	$ 831	$ 2,982	$ 2,493