Pension Plans (Plan Assets for Qualified Pension Plan) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		$ (643,392)	$ (572,174)
Not subject to leveling		(314,469)	(216,354)
Defined benefit plan, fair value plan assets before accrued income		642,350	570,647
Alternative Investment before Accrued Interest, Fair Value Disclosure		314,420
Defined benefit plan, fair value of plan assets, accrued income		1,042	1,527
Alternative Investment, Accrued Interest		49
Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(233,000)	(227,915)
Defined benefit plan, fair value plan assets before accrued income		232,880	227,782
Defined benefit plan, fair value of plan assets, accrued income		120	133
Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(95,923)	(127,905)
Defined benefit plan, fair value plan assets before accrued income		95,050	126,511
Defined benefit plan, fair value of plan assets, accrued income		873	1,394
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		(35,046)	(21,537)
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		(35,046)	(21,537)
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
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(96,318)	(127,857)
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		(96,318)	(127,857)
Fixed income securities | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value			0
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(126,955)	(131,897)
Not subject to leveling		0	0
Mutual funds | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(126,955)	(131,897)
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
Common trust funds
Defined Benefit Plan Disclosure [Line Items]
Total carrying value	[1]	(314,420)	(216,354)
Not subject to leveling	[1]	(314,420)	(216,354)
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
Corporate stock
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(70,879)	(74,348)
Not subject to leveling		0	0
Corporate stock | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(70,879)	(74,348)
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
Assets | Futures contracts - liabilities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(116)
Not subject to leveling		0
Assets | Futures contracts - liabilities | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0
Assets | Futures contracts - liabilities | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(116)
Assets | Futures contracts - liabilities | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0
Liability | Futures contracts - liabilities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(1,384)	(1,346)
Not subject to leveling		0	0
Liability | Futures contracts - liabilities | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Liability | Futures contracts - liabilities | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(1,384)	(1,346)
Liability | Futures contracts - liabilities | 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.