Pension Plans (Plan Assets for Qualified Pension Plan) (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		$ 572,174	$ 553,858
Not subject to leveling		216,354	196,570
Defined benefit plan, fair value plan assets before accrued income		570,647	551,872
Defined benefit plan, fair value of plan assets, accrued income		1,527	1,986
Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		227,915	180,739
Defined benefit plan, fair value plan assets before accrued income		227,782	180,666
Defined benefit plan, fair value of plan assets, accrued income		133	73
Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		127,905	176,549
Defined benefit plan, fair value plan assets before accrued income		126,511	174,636
Defined benefit plan, fair value of plan assets, accrued income		1,394	1,913
Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Defined benefit plan, fair value plan assets before accrued income		0	0
Defined benefit plan, fair value of plan assets, accrued income		0	0
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		21,537	10,201
Not subject to leveling		0	0
Cash and cash equivalents | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		21,537	10,201
Cash and cash equivalents | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Cash and cash equivalents | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Futures contracts - assets/(liabilities)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(1,346)
Not subject to leveling		0
Futures contracts - assets/(liabilities) | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0
Futures contracts - assets/(liabilities) | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(1,346)
Futures contracts - assets/(liabilities) | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		127,857	174,636
Not subject to leveling		0	0
Fixed income securities | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Fixed income securities | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		127,857	174,636
Fixed income securities | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		131,897	119,312
Not subject to leveling		0	0
Mutual funds | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		131,897	119,312
Mutual funds | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Mutual funds | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Common trust funds
Defined Benefit Plan Disclosure [Line Items]
Total carrying value	[1]	216,354	196,570
Not subject to leveling	[1]	216,354	196,570
Common trust funds | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Common trust funds | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Common trust funds | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Corporate stock
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		74,348	51,153
Not subject to leveling		0	0
Corporate stock | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		74,348	51,153
Corporate stock | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Corporate stock | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		$ 0	$ 0

[1] 1 In accordance with ASU 2015-07, Fair Value Measurement (Topic 820), certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented for the total pension plan assets.