Employee Benefits (Fair Value Measurements- Other Postretirement Benefits) (Details) - USD ($)			12 Months Ended
		Jun. 30, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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
Other Postretirement Benefit 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, 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
Fair value of plan assets at end of year			41,811,000	36,287,000	$ 40,873,000
Limited Partnership expiration date		Jun. 30, 2016
Cash and Cash Equivalents | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			2,522,000	1,353,000
Institutional Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		38,950,000	34,305,000
Limited Partnership Interest in Real Estate | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[2]		339,000	629,000	$ 722,000
Fair Value, Inputs, Level 1 | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			41,472,000	35,658,000
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year			2,522,000	1,353,000
Fair Value, Inputs, Level 1 | Institutional Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		38,950,000	34,305,000
Fair Value, Inputs, Level 2 | Other Postretirement Benefit 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 | Other Postretirement Benefit 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 | Institutional Funds | Other Postretirement Benefit 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 | Other Postretirement Benefit 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 | Other Postretirement Benefit 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 | Institutional Funds | Other Postretirement Benefit 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
Institutional Funds | Equity Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		18,664,000	17,887,000
Institutional Funds | Fixed Income Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		15,038,000	11,437,000
Institutional Funds | Multi Asset Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		3,766,000	3,576,000
Institutional Funds | Real Asset Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		1,482,000	1,405,000
Institutional Funds | Fair Value, Inputs, Level 1 | Equity Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		18,664,000	17,887,000
Institutional Funds | Fair Value, Inputs, Level 1 | Fixed Income Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		15,038,000	11,437,000
Institutional Funds | Fair Value, Inputs, Level 1 | Multi Asset Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		3,766,000	3,576,000
Institutional Funds | Fair Value, Inputs, Level 1 | Real Asset Funds | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets at end of year	[1]		1,482,000	1,405,000
Institutional Funds | Fair Value, Inputs, Level 2 | Equity Funds | Other Postretirement Benefit 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
Institutional Funds | Fair Value, Inputs, Level 2 | Fixed Income Funds | Other Postretirement Benefit 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
Institutional Funds | Fair Value, Inputs, Level 2 | Multi Asset Funds | Other Postretirement Benefit 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
Institutional Funds | Fair Value, Inputs, Level 2 | Real Asset Funds | Other Postretirement Benefit 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
Institutional Funds | Fair Value, Inputs, Level 3 | Equity Funds | Other Postretirement Benefit 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
Institutional Funds | Fair Value, Inputs, Level 3 | Fixed Income Funds | Other Postretirement Benefit 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
Institutional Funds | Fair Value, Inputs, Level 3 | Multi Asset Funds | Other Postretirement Benefit 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
Institutional Funds | Fair Value, Inputs, Level 3 | Real Asset Funds | Other Postretirement Benefit 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 institutional funds are invested in equity or 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] This investment is a commercial real estate partnership that purchases land, develops limited infrastructure and sells it for commercial development. The OPEB Plan trust was restricted from selling its partnership interest during the life of the partnership, which spanned 7 years. Return of 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.