Equity Offering (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Sep. 30, 2013	Sep. 16, 2013	May 08, 2012
Equity Offering [Line Items]
Proceeds from sale of common stock	$ 0	$ 174,670,000	$ 79,040,000
Senior secured notes [Member]
Equity Offering [Line Items]
Proceeds Used for Early Extinguishment of Debt				54,900,000
Extinguishment of Debt, Amount				50,000,000
Common Stock [Member]
Equity Offering [Line Items]
Stock Issued During Period, Shares, New Issues				4,000,000	5,175,000
Stock Issued During Period, Value, Per Share Offering Price						$ 35.40	$ 20.75
Stock Issuance Costs				4,000,000	8,500,000
Proceeds from sale of common stock				$ 79,000,000	$ 174,700,000