Derivatives (Interest rate swap agreements) (Details) $ in Millions	3 Months Ended
	Jun. 30, 2018 USD ($)
Swap Agreement Three [Member]
Swaps:
Debt instrument, face amount	$ 300.0
Derivative, Inception Date	Aug. 16, 2006
Derivative Effective Date	Aug. 18, 2006
Derivative, Maturity Date	Aug. 10, 2018
Designated Cash Flow Hedge Date	Aug. 04, 2006
Swap Agreement Thirteen [Member]
Swaps:
Debt instrument, face amount	$ 50.0	[1]
Derivative, Inception Date	Jul. 29, 2011
Derivative Effective Date	Aug. 15, 2011
Derivative, Maturity Date	Aug. 15, 2018
Designated Cash Flow Hedge Date	Jul. 29, 2011
Swap Agreement Fourteen [Member]
Swaps:
Debt instrument, face amount	$ 20.0	[1]
Derivative, Inception Date	Aug. 03, 2011
Derivative Effective Date	Sep. 12, 2011
Derivative, Maturity Date	Sep. 10, 2018
Designated Cash Flow Hedge Date	Aug. 03, 2011
Swap Agreement Fifteen [Member]
Swaps:
Debt instrument, face amount	$ 15.1	[2]
Derivative, Inception Date	Mar. 27, 2012
Derivative Effective Date	Mar. 28, 2012
Derivative, Maturity Date	Mar. 28, 2019
Designated Cash Flow Hedge Date	Mar. 26, 2012
Swap Agreement Sixteen [Member]
Swaps:
Debt instrument, face amount	$ 25.0
Derivative, Inception Date	Apr. 13, 2012
Derivative Effective Date	Apr. 16, 2012
Derivative, Maturity Date	Apr. 01, 2019
Designated Cash Flow Hedge Date	Apr. 12, 2012
Swap Agreement Seventeen [Member]
Swaps:
Debt instrument, face amount	$ 44.3
Derivative, Inception Date	Jan. 11, 2013
Derivative Effective Date	Jan. 15, 2013
Derivative, Maturity Date	Dec. 15, 2019
Designated Cash Flow Hedge Date	Jan. 11, 2013

[1] forward swap

[2] operating lease