Derivatives (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Sep. 30, 2012
Swap Agreement One [Member]
Swaps:
Original variable, rate debt amount	$ 50
Agreement date	Jun. 21, 2006
Effective date	Jul. 10, 2006
Expiration date	Jul. 10, 2013
Designed cash flow hedge date	Jun. 09, 2006
Swap Agreement Two [Member]
Swaps:
Original variable, rate debt amount	145	[1]
Agreement date	Jun. 09, 2006
Effective date	Oct. 10, 2006
Expiration date	Oct. 10, 2012
Designed cash flow hedge date	Jun. 09, 2006
Swap Agreement Three [Member]
Swaps:
Original variable, rate debt amount	300
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 Four [Member]
Swaps:
Original variable, rate debt amount	30
Agreement date	Feb. 09, 2007
Effective date	Feb. 12, 2007
Expiration date	Feb. 10, 2014
Designed cash flow hedge date	Feb. 09, 2007
Swap Agreement Five [Member]
Swaps:
Original variable, rate debt amount	20
Agreement date	Mar. 08, 2007
Effective date	Mar. 12, 2007
Expiration date	Mar. 10, 2014
Designed cash flow hedge date	Mar. 08, 2007
Swap Agreement Six [Member]
Swaps:
Original variable, rate debt amount	20
Agreement date	Mar. 08, 2007
Effective date	Mar. 12, 2007
Expiration date	Mar. 10, 2014
Designed cash flow hedge date	Mar. 08, 2007
Swap Agreement Seven [Member]
Swaps:
Original variable, rate debt amount	19	[1]
Agreement date	Apr. 08, 2008
Effective date	Aug. 15, 2008
Expiration date	Jun. 15, 2015
Designed cash flow hedge date	Mar. 31, 2008
Swap Agreement Eight [Member]
Swaps:
Original variable, rate debt amount	19
Agreement date	Aug. 27, 2008
Effective date	Aug. 29, 2008
Expiration date	Jul. 10, 2015
Designed cash flow hedge date	Apr. 10, 2008
Swap Agreement Nine [Member]
Swaps:
Original variable, rate debt amount	30
Agreement date	Sep. 24, 2008
Effective date	Sep. 30, 2008
Expiration date	Sep. 10, 2015
Designed cash flow hedge date	Sep. 24, 2008
Swap Agreement Ten [Member]
Swaps:
Original variable, rate debt amount	15	[1]
Agreement date	Mar. 24, 2009
Effective date	Mar. 30, 2009
Expiration date	Mar. 30, 2016
Designed cash flow hedge date	Mar. 25, 2009
Swap Agreement Eleven [Member]
Swaps:
Original variable, rate debt amount	15	[1]
Agreement date	Jul. 06, 2010
Effective date	Aug. 15, 2010
Expiration date	Jul. 15, 2017
Designed cash flow hedge date	Jul. 06, 2010
Swap Agreement Twelve [Member]
Swaps:
Original variable, rate debt amount	25	[1]
Agreement date	Apr. 26, 2011
Effective date	Jun. 01, 2011
Expiration date	Jun. 01, 2018
Designed cash flow hedge date	Jul. 01, 2011
Swap Agreement Thirteen [Member]
Swaps:
Original variable, rate debt amount	50	[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 [Member]
Swaps:
Original variable, rate debt amount	20	[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 [Member]
Swaps:
Original variable, rate debt amount	15	[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 [Member]
Swaps:
Original variable, rate debt amount	$ 25
Agreement date	Apr. 13, 2012
Effective date	Apr. 16, 2012
Expiration date	Apr. 01, 2019
Designed cash flow hedge date	Apr. 12, 2012

[1] forward swap

[2] operating lease