Debt and Credit Arrangements Committed and Uncommitted Credit Facility (Details) $ in Millions	3 Months Ended
	Jun. 30, 2017 USD ($)
Committed credit facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, expiration date	Oct. 20, 2020
Line of credit facility, current borrowing capacity	$ 1,000.0
Line of credit facility, available capacity increase amount	250.0
Line of credit facility, limits on letters of credit	200.0
Line of credit outstanding, amount	0.0
Letters of credit outstanding, amount	8.4
Line of credit facility, remaining borrowing capacity	991.6
Uncommitted credit facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, current borrowing capacity	906.5
Line of credit outstanding, amount	237.7
Line of credit facility, average outstanding amount	$ 457.6
Line of credit facility, weighted average interest rate	2.20%