Common Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended						12 Months Ended									16 Months Ended	6 Months Ended	14 Months Ended	6 Months Ended	14 Months Ended
	Nov. 30, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 17, 2012	Feb. 27, 2012	Sep. 07, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Restricted Common Stock [Member]	Dec. 31, 2012 Common Stock	Jul. 09, 2012 Common Stock	Jun. 18, 2012 Common Stock	Nov. 07, 2011 Common Stock	Nov. 02, 2011 Common Stock	Sep. 07, 2011 Common Stock	Dec. 31, 2012 IPO [Member]	Dec. 31, 2012 Follow on Offering [Member]	Dec. 31, 2012 Follow on Offering [Member]	Dec. 31, 2012 Underwriters' over allotment [Member]	Dec. 31, 2012 Underwriters' over allotment [Member]	Dec. 31, 2012 Registration Payment Arrangement, Arrangement [Domain]
Class of Stock [Line Items]
Common Stock, Shares, Issued							11,157,643	7,323,434		10,884,534	487,500	3,250,000	74,979	1,497,924	5,574,131						1,882,248
Proceeds from Issuance of Common Stock							$ 118.9									$ 67.4	$ 30.3	$ 15.8	$ 4.6	$ 0.8
Common Stock, Shares, Outstanding							11,157,643	7,323,434	0
Dividends declared, in dollars per share	$ 0.9000	$ 0.8950	$ 0.8900	$ 0.8850	$ 0.8800	$ 0.8750