Employee Benefit Plans Employee Benefit Plans 2 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Change in Projected Benefit Obligation:
Projected benefit obligation at beginning of year	$ 180,364	$ 168,934
Service cost	4,999	5,094	$ 6,276
Interest cost	7,904	7,910	7,686
Actuarial (gain) loss	20,397	4,162
Benefits/expenses paid	(5,974)	(5,736)
Projected benefit obligation at end of year	207,690	180,364	168,934
Changes in Plan Assets:
Fair value of plan assets at beginning of year	150,872	142,174
Actual return on plan assets	22,246	9,182
Employer contributions	6,504	5,252
Benefits/expenses paid	5,974	5,736
Fair value of plan assets at end of year	173,648	150,872	142,174
Funded Status:
Net amount recognized as accrued pension cost	(34,042)	(29,492)
Post-Retirement Medical Benefits
Change in Projected Benefit Obligation:
Projected benefit obligation at beginning of year	8,802	9,393
Service cost	227	247	340
Interest cost	324	371	435
Actuarial (gain) loss	(355)	(715)
Benefits/expenses paid	(507)	(494)
Projected benefit obligation at end of year	8,491	8,802	9,393
Changes in Plan Assets:
Fair value of plan assets at beginning of year	10,538	10,614
Actual return on plan assets	1,022	418
Employer contributions	0	0
Benefits/expenses paid	507	494
Fair value of plan assets at end of year	11,053	10,538	$ 10,614
Funded Status:
Net amount recognized as accrued pension cost	$ 2,562	$ 1,736