Components of Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,780	$ 1,547	$ 3,560	$ 3,094
Interest cost	2,129	2,024	4,258	4,048
Expected return on plan assets	(4,815)	(5,677)	(9,630)	(11,354)
Amortization of Prior service cost	0	0	0	0
Amortization of Net loss	707	2,123	1,414	4,246
Net periodic cost (income)	(199)	17	(398)	34
RIM
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	61	35	122	70
Interest cost	111	96	222	192
Expected return on plan assets	0	0	0	0
Amortization of Prior service cost	0	0	0	0
Amortization of Net loss	103	71	206	142
Net periodic cost (income)	275	202	550	404
Post-retirement
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	93	112	186	224
Interest cost	205	214	410	428
Expected return on plan assets	0	0	0	0
Amortization of Prior service cost	(34)	(34)	(68)	(68)
Amortization of Net loss	69	85	138	170
Net periodic cost (income)	$ 333	$ 377	$ 666	$ 754