Employee Benefits - Plan Assets and Investment Policy (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Employee Benefits
Weighted-average asset allocations (as a percent)	100.00%	100.00%
Fair value of plan assets	$ 73.3	$ 70.2
Level 1
Employee Benefits
Fair value of plan assets	52.7	50.3
Level 2
Employee Benefits
Fair value of plan assets	20.6	19.9
Equity securities
Employee Benefits
Fair value of plan assets	23.5	24.8
Equity securities \| Level 1
Employee Benefits
Fair value of plan assets	23.5	24.8
U.S. government, state, and agency
Employee Benefits
Fair value of plan assets	7.3	7.9
U.S. government, state, and agency \| Level 2
Employee Benefits
Fair value of plan assets	7.3	7.9
Corporate debt securities
Employee Benefits
Fair value of plan assets	10.3	9.7
Corporate debt securities \| Level 2
Employee Benefits
Fair value of plan assets	10.3	9.7
Mutual funds
Employee Benefits
Fair value of plan assets	29.3	23.4
Mutual funds \| Level 1
Employee Benefits
Fair value of plan assets	26.3	21.1
Mutual funds \| Level 2
Employee Benefits
Fair value of plan assets	3.0	2.3
Interest and non-interest bearing cash
Employee Benefits
Fair value of plan assets	2.9	4.4
Interest and non-interest bearing cash \| Level 1
Employee Benefits
Fair value of plan assets	$ 2.9	$ 4.4
Equity securities
Employee Benefits
Weighted-average asset allocations (as a percent)	58.00%	56.00%
Target allocation
Target allocation, minimum (as a percent)	50.00%
Target allocation, maximum (as a percent)	80.00%
Debt securities
Employee Benefits
Weighted-average asset allocations (as a percent)	38.00%	38.00%
Target allocation
Target allocation, minimum (as a percent)	20.00%
Target allocation, maximum (as a percent)	60.00%
Other
Employee Benefits
Weighted-average asset allocations (as a percent)	4.00%	6.00%
Target allocation
Target allocation, minimum (as a percent)	0.00%
Target allocation, maximum (as a percent)	10.00%

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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 30
-Section 35
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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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