Subsequent Events (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2012
Issuance of Debt [Member]
Subsequent Event [Line Items]
Subsequent event, Date	Jul. 05, 2012
Subsequent event, Amount	$ 168.0
Subsequent event, Description	We concluded a private placement of senior unsecured notes, divided into three tranches - $42.0 million 3.67% Series A Notes due 2022, $53.0 million 3.82% Series B Notes due 2024 and $73.0 million 4.02% Series C Notes due 2027.
Weighted average interest rate	3.87%
Annual effective rate of interest	4.16%
Refinancing of Debt [Member]
Subsequent Event [Line Items]
Subsequent event, Date	Jul. 30, 2012
Subsequent event, Amount	$ 50.0
Subsequent event, Description	We used a portion of our revolving credit facility to repay our $50.0 million Series A floating rate notes that matured on July 28, 2012.