Employee Benefits - Plan Assets and Investment Policy (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Employee Benefits
Weighted-average asset allocations (as a percent)	100.00%	100.00%
Fair value of plan assets	$ 94.9	$ 89.0
Level 1
Employee Benefits
Fair value of plan assets	63.3	63.4
Level 2
Employee Benefits
Fair value of plan assets	31.6	25.6
Equity securities
Employee Benefits
Fair value of plan assets	27.5	26.7
Equity securities \| Level 1
Employee Benefits
Fair value of plan assets	27.5	26.7
U.S. government, state, and agency
Employee Benefits
Fair value of plan assets	11.6	4.7
U.S. government, state, and agency \| Level 2
Employee Benefits
Fair value of plan assets	11.6	4.7
Corporate debt securities
Employee Benefits
Fair value of plan assets	17.4	17.4
Corporate debt securities \| Level 2
Employee Benefits
Fair value of plan assets	17.4	17.4
Mutual funds
Employee Benefits
Fair value of plan assets	34.8	34.3
Mutual funds \| Level 1
Employee Benefits
Fair value of plan assets	32.2	30.8
Mutual funds \| Level 2
Employee Benefits
Fair value of plan assets	2.6	3.5
Interest and non-interest bearing cash
Employee Benefits
Fair value of plan assets	3.6	5.9
Interest and non-interest bearing cash \| Level 1
Employee Benefits
Fair value of plan assets	$ 3.6	$ 5.9
Equity securities
Employee Benefits
Weighted-average asset allocations (as a percent)	51.00%	55.00%
Equity securities \| Minimum
Target allocation
Target allocation (as a percent)	35.00%
Equity securities \| Maximum
Target allocation
Target allocation (as a percent)	65.00%
Debt securities
Employee Benefits
Weighted-average asset allocations (as a percent)	45.00%	38.00%
Debt securities \| Minimum
Target allocation
Target allocation (as a percent)	15.00%
Debt securities \| Maximum
Target allocation
Target allocation (as a percent)	45.00%
Other
Employee Benefits
Weighted-average asset allocations (as a percent)	4.00%	7.00%
Other \| Minimum
Target allocation
Target allocation (as a percent)	0.00%
Other \| Maximum
Target allocation
Target allocation (as a percent)	15.00%

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+ References

No definition available.

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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
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