Subsequent Events (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Subsequent Events [Line Items]
Net proceeds of sale or issuance of equity, percentage	50.00%
Net proceeds of incurrence of certain indebtedness, percentage	50.00%
Reduction to total loan commitments in event of rating decline	$ 300
April 17, 2012 Credit Agreement [Member]
Subsequent Events [Line Items]
Line of credit facility, initiation date	April 17, 2012
Line of credit, borrowing capacity	600.0
Funds borrowed under new agreement	200.0
Revolving credit facility, termination date	April 17, 2016
April 12, 2010 Credit Agreement [Member]
Subsequent Events [Line Items]
Line of credit facility, initiation date	April 12, 2010
Line of credit, borrowing capacity	400.0
Payment on outstanding balance of prior agreement	$ 200.0
S&P Rating [Member]
Subsequent Events [Line Items]
Senior, unsecured, non-credit enhanced long-term debt, lower credit quality rating	BBB-
Moody's Rating [Member]
Subsequent Events [Line Items]
Senior, unsecured, non-credit enhanced long-term debt, lower credit quality rating	Baa3
Maximum [Member] | Alternate Base Rate Borrowings [Member]
Subsequent Events [Line Items]
Borrowings under the credit agreement	1.50%
Maximum [Member] | Adjusted LIBOR Rate Borrowings [Member]
Subsequent Events [Line Items]
Borrowings under the credit agreement	2.50%
Minimum [Member] | Alternate Base Rate Borrowings [Member]
Subsequent Events [Line Items]
Borrowings under the credit agreement	0.75%
Minimum [Member] | Adjusted LIBOR Rate Borrowings [Member]
Subsequent Events [Line Items]
Borrowings under the credit agreement	1.75%