Derivatives (Interest rate swap agreements) (Details) $ in Millions	9 Months Ended
	Dec. 31, 2017 USD ($)
Swap Agreement Three
Swaps:
Original variable, rate debt amount	$ 300.0
Agreement date	Aug. 16, 2006
Effective date	Aug. 18, 2006
Expiration date	Aug. 10, 2018
Designed cash flow hedge date	Aug. 04, 2006
Swap Agreement Twelve
Swaps:
Original variable, rate debt amount	$ 25.0	[1]
Agreement date	Apr. 26, 2011
Effective date	Jun. 01, 2011
Expiration date	Jun. 01, 2018
Designed cash flow hedge date	Jun. 01, 2011
Swap Agreement Thirteen
Swaps:
Original variable, rate debt amount	$ 50.0	[1]
Agreement date	Jul. 29, 2011
Effective date	Aug. 15, 2011
Expiration date	Aug. 15, 2018
Designed cash flow hedge date	Jul. 29, 2011
Swap Agreement Fourteen
Swaps:
Original variable, rate debt amount	$ 20.0	[1]
Agreement date	Aug. 03, 2011
Effective date	Sep. 12, 2011
Expiration date	Sep. 10, 2018
Designed cash flow hedge date	Aug. 03, 2011
Swap Agreement Fifteen
Swaps:
Original variable, rate debt amount	$ 15.1	[2]
Agreement date	Mar. 27, 2012
Effective date	Mar. 28, 2012
Expiration date	Mar. 28, 2019
Designed cash flow hedge date	Mar. 26, 2012
Swap Agreement Sixteen
Swaps:
Original variable, rate debt amount	$ 25.0
Agreement date	Apr. 13, 2012
Effective date	Apr. 16, 2012
Expiration date	Apr. 01, 2019
Designed cash flow hedge date	Apr. 12, 2012
Swap Agreement Seventeen
Swaps:
Original variable, rate debt amount	$ 44.3
Agreement date	Jan. 11, 2013
Effective date	Jan. 15, 2013
Expiration date	Dec. 15, 2019
Designed cash flow hedge date	Jan. 11, 2013

[1] forward swap

[2] operating lease