Employee Benefits (Fair Value Measurement Inputs) (Details) - USD ($)			12 Months Ended
		Jun. 30, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Description			no		no
Fair Value, Assets, Level 2 to Level 1 Transfers, Description			no		no
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, Sales, Issuances, Settlements			$ 0		$ 0
Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Restriction Period To Sell Partnership Interest Maximum			7 years
Fair value of plan assets at end of year			$ 304,389,000		$ 269,766,000		$ 260,035,000
Fair Value, Assets, Level 1 to Level 2 Transfers, Description			no		no
Fair Value, Assets, Level 2 to Level 1 Transfers, Description			no		no
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, Sales, Issuances, Settlements			$ 0		$ 0
Limited Partnership expiration date		Jun. 30, 2016
Cash and Cash Equivalents | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			1,582,000		932,000
Common Collective Trusts | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		298,954,000		261,843,000
Limited Partnership Interest in Real Estate | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			3,853,000	[2],[3]	6,991,000	[2],[3]	$ 8,588,000
Fair Value, Inputs, Level 1 | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			300,536,000		262,775,000
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			1,582,000		932,000
Fair Value, Inputs, Level 1 | Common Collective Trusts | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		298,954,000		261,843,000
Fair Value, Inputs, Level 2 | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			0		0
Fair Value, Inputs, Level 2 | Cash and Cash Equivalents | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			0		0
Fair Value, Inputs, Level 2 | Common Collective Trusts | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		0		0
Fair Value, Inputs, Level 3 | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			0		0
Fair Value, Inputs, Level 3 | Cash and Cash Equivalents | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			0		0
Fair Value, Inputs, Level 3 | Common Collective Trusts | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		0		0
Common Collective Trusts | Equity Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		158,684,000		144,081,000
Common Collective Trusts | Fixed Income Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		124,491,000		109,356,000
Common Collective Trusts | Real Estate Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		15,779,000		8,406,000
Common Collective Trusts | Fair Value, Inputs, Level 1 | Equity Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		158,684,000		144,081,000
Common Collective Trusts | Fair Value, Inputs, Level 1 | Fixed Income Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		124,491,000		109,356,000
Common Collective Trusts | Fair Value, Inputs, Level 1 | Real Estate Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		15,779,000		8,406,000
Common Collective Trusts | Fair Value, Inputs, Level 2 | Equity Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		0		0
Common Collective Trusts | Fair Value, Inputs, Level 2 | Fixed Income Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		0		0
Common Collective Trusts | Fair Value, Inputs, Level 2 | Real Estate Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		0		0
Common Collective Trusts | Fair Value, Inputs, Level 3 | Equity Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		0		0
Common Collective Trusts | Fair Value, Inputs, Level 3 | Fixed Income Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		0		0
Common Collective Trusts | Fair Value, Inputs, Level 3 | Real Estate Funds | Retirement Income Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		$ 0		$ 0

[1] The Common Collective Trusts are invested in equity and fixed income securities, or a combination thereof. The investment objective of each fund is to produce returns in excess of, or commensurate with, its predefined index.

[2] In the first quarter of 2016, the Company implemented ASU 2015-07, Fair Value Measurement (Topic 820) which eliminates the requirement to categorize investments in the fair value hierarchy if the fair value is measured at NAV per share (or its equivalent) using the practical expedient in the FASB’s fair value measurement guidance. The fair value amounts presented in the table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the statement of financial position.

[3] This investment is a commercial real estate partnership that purchases land, develops limited infrastructure, and sells it for commercial development. The Company was restricted from selling its partnership interest during the life of the partnership, which spanned 7 years. Return on investment is realized as land is sold. The fair value of the limited partnership interest in real estate is based on the NAV of the partnership which reflects the appraised value of the land. The partnership term expired on June 30, 2016. Upon expiration, dissolution of the partnership commenced and, as a result, the general partner of the partnership is attempting to sell the remaining inventory as soon as possible at the highest pricing possible.