Note 11 - Revolving Credit Facility and Letter of Credit Facilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Oct. 19, 2012
Cash Collateral Deposits	$ 27.1
Matures in October 2015 [Member] | Unsecured Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	320
Matures in February 2014 [Member] | Unsecured Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	30
Unsecured Revolving Credit Facility [Member]
Line of Credit Facility, Maximum Borrowing Capacity	350
Aggregate Borrowing Commitment		550
Line of Credit Facility, Current Borrowing Capacity	350.0
Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	11
Letters of Credit Outstanding, Amount	7.2
Uncommitted Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	30
Letters of Credit Outstanding, Amount	$ 19.5