Stockholders' Equity (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stockholders' Equity
Number of shares issued	4,600,000	0
Dividend rate on preferred stock (as a percent)	7.25%	7.25%
Net Proceeds from issuance of preferred stock	$ 110,615
Net Proceeds from issuance of common stock			310,960
Liquidation preference on preferred stock (in dollars per share)	$ 25.00	$ 25.00
Series A Cumulative Redeemable PreferredStock
Stockholders' Equity
Number of shares issued	4,600,000
Dividend rate on preferred stock (as a percent)	7.25%
Net Proceeds from issuance of preferred stock	$ 110,600
Cumulative dividends each year on preferred stock (in dollars per share)	$ 1.8125
Liquidation preference on preferred stock (in dollars per share)	$ 25.00
Dividend payable on preferred stock (in dollars per share)	$ 0.6193
Redemption price of preferred stock (in dollars per share)	$ 25.00
Minimum Period during which no payments of dividend of preferred stock leads to entitlement of voting rights	1 year 6 months
Conversion ratio of preferred stock	1.944