Common Stock (Details) (USD $)	6 Months Ended				3 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011		Jun. 30, 2012 May 2012 Common Stock Issuance [Member]	May 21, 2012 May 2012 Common Stock Issuance [Member]
Class of Stock [Line Items]
Additional paid-in capital	$ 384,275,000		$ 217,707,000	[1]		$ 163,900,000
Equity Issuance, Date					May 21, 2012
Stock Issued During Period, Shares, New Issues					6,500,000
Sale of Stock, Price Per Share						$ 26.50
Proceeds from Issuance of Common Stock	164,050,000	0			164,000,000
Common shares, par value $0.01 per share, authorized 100,000,000 shares; issued: 30,261,150 in 2012 and 23,634,958 in 2011	$ 303,000		$ 236,000	[1]		$ 65,000

[1] Derived from audited 2011 balance sheet.