Common Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended					1 Months Ended	3 Months Ended						0 Months Ended				6 Months Ended	12 Months Ended
	Jun. 30, 2012	Jun. 08, 2012	Mar. 15, 2012	Feb. 27, 2012	Sep. 07, 2011	Jul. 31, 2012	Jun. 27, 2012	Jun. 18, 2012	Dec. 31, 2011	Nov. 07, 2011	Nov. 02, 2011	Sep. 06, 2011	Jun. 18, 2012 Common Stock [Member]	Nov. 02, 2011 Common Stock [Member]	Nov. 07, 2011 Common Stock [Member]	Sep. 06, 2011 Common Stock [Member]	Jun. 30, 2012 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	10,676,117								7,323,434
Dividends declared, in dollars per share				$ 0.880	$ 0.875	$ 0.885	$ 0.885
Common stock, dividends, per share per month, declared	$ 0.890
Dividends paid per share		$ 0.07375	$ 0.0733
Common stock, shares issued	10,676,117							3,250,000	7,323,434	100,000	1,500,000	5,600,000
Proceeds from Issuance of Common Stock													$ 30.0	$ 14.4	$ 0.7	$ 66.0	$ 108.1	$ 78.1