Stockholders' Equity (Details 1) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jan. 31, 2012 Dividend Declared [Member]
Subsequent Event [Line Items]
Subsequent Event Per Share					$ 0.30
Schedule of sale of stock under market equity offering Program
Sold Shares of common stock under equity offering program		11,648,700	5,258,700	1,201,600
Minimum price of common stock under equity offering program		$ 20.27	$ 20.23	$ 21.62
Maximum price of common stock under equity offering program		$ 23.63	$ 25.16	$ 22.50
Proceeds from issuance of common stock under at-the-market equity offering program		$ 251.6	$ 117.7	$ 25.7
Stockholders Equity (Textual) [Abstract]
Reduction to future benefit obligations		1.9
Dividends per share to common stockholders		$ 1.20
Dividends declared per share to common stockholders per quarter	$ 0.30	$ 1.20	$ 1.20	$ 1.20
Number of common shares authorized to be repurchased	3,000,000	3,000,000
Increase in future benefit obligation due to increase in other liabilities		0.7	0.7
Decrease in future benefit obligation due to decrease in other liabilities		2.6	2.6
Other comprehensive income loss reduction to future benefit obligations related to pension plan		$ 1.9