Derivatives (Narrative) (Details) gal in Millions, $ in Millions	3 Months Ended
Mar. 31, 2021 USD ($) gal	Dec. 31, 2020 USD ($)
Derivative [Line Items]
Value of trading portfolio realized - gains (losses) \| $		$ (0.2)
Current asset representing cash collateral posted \| $	$ 2.6	$ 1.4
Not Designated as Hedging Instrument \| Diesel Purchases \| Minimum
Derivative [Line Items]
Diesel fuel purchased annually (in gallons) \| gal	30
Not Designated as Hedging Instrument \| Diesel Purchases \| Maximum
Derivative [Line Items]
Diesel fuel purchased annually (in gallons) \| gal	35

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=d3e41678-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Name:	us-gaap_DerivativeCollateralObligationToReturnCash
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant