Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 34.8	$ 24.3	$ 48.8	$ 30.2
Other comprehensive income:
Amortization of actuarial losses and prior service credits, net of income taxes of $4.2 million, $6.2 million, $8.4 million, and $12.4 million	12.5	9.4	25.0	19.1
Comprehensive income	47.3	33.7	73.8	49.3
Income taxes related to defined benefit plan	$ 4.2	$ 6.2	$ 8.4	$ 12.4

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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