Revenue - Change in Contract Assets and Liabilities (Details) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($)
Contract assets
Beginning accounts receivable, net of allowances	$ 200.8
Change in accounts receivable	19.2
Ending accounts receivable, net of allowances	220.0
Beginning unbilled contract revenue	79.4
Change in unbilled contract revenue	(11.2)
Ending unbilled contract revenue	$ 68.2
Change in accounts receivable (as a percentage)	9.60%
Change in unbilled contract revenue (as a percentage)	(14.10%)
Contract liabilities
Beginning balance customer advances and billings in excess of contract revenue	$ 118.9
Change in customer advances and billings in excess of contract revenue	7.2
Ending balance customer advances and billings in excess of contract revenue	126.1
Beginning long-term deferred revenue	1.9
Change in long-term deferred revenue	(0.9)
Ending long-term deferred revenue	$ 1.0
Change in customer advances and billings in excess of contract revenue (as a percentage)	6.10%
Change in long-term deferred revenue (as a percentage)	(47.40%)