Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 271.5	$ 265.7	$ 245.2
Other comprehensive loss, net of tax:
Loss on cash flow hedges, net of tax benefit of $0.3, $0 and $0	(0.7)
Loss on cash flow hedges, net of tax benefit of $0.3, $0 and $0		0.0	0.0
Comprehensive income	$ 270.8	$ 265.7	$ 245.2

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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