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Employee Benefits - Plan Assets and Investment Policy (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Employee Benefits
Weighted-average asset allocations (as a percent)	100.00%	100.00%
Fair value of plan assets	$ 89.0	$ 73.3
Level 1
Employee Benefits
Fair value of plan assets	63.4	52.7
Level 2
Employee Benefits
Fair value of plan assets	25.6	20.6
Equity securities
Employee Benefits
Fair value of plan assets	26.7	23.5
Equity securities | Level 1
Employee Benefits
Fair value of plan assets	26.7	23.5
U.S. government, state, and agency
Employee Benefits
Fair value of plan assets	4.7	7.3
U.S. government, state, and agency | Level 2
Employee Benefits
Fair value of plan assets	4.7	7.3
Corporate debt securities
Employee Benefits
Fair value of plan assets	17.4	10.3
Corporate debt securities | Level 2
Employee Benefits
Fair value of plan assets	17.4	10.3
Mutual funds
Employee Benefits
Fair value of plan assets	34.3	29.3
Mutual funds | Level 1
Employee Benefits
Fair value of plan assets	30.8	26.3
Mutual funds | Level 2
Employee Benefits
Fair value of plan assets	3.5	3.0
Interest and non-interest bearing cash
Employee Benefits
Fair value of plan assets	5.9	2.9
Interest and non-interest bearing cash | Level 1
Employee Benefits
Fair value of plan assets	$ 5.9	$ 2.9
Equity securities
Employee Benefits
Weighted-average asset allocations (as a percent)	55.00%	58.00%
Target allocation
Target allocation, minimum (as a percent)	35.00%
Target allocation, maximum (as a percent)	65.00%
Debt securities
Employee Benefits
Weighted-average asset allocations (as a percent)	38.00%	38.00%
Target allocation
Target allocation, minimum (as a percent)	15.00%
Target allocation, maximum (as a percent)	45.00%
Other
Employee Benefits
Weighted-average asset allocations (as a percent)	7.00%	4.00%
Target allocation
Target allocation, minimum (as a percent)	0.00%
Target allocation, maximum (as a percent)	15.00%

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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 30
-Section 35
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-Publisher FASB
-Name Accounting Standards Codification
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Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

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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.

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