Summary of Significant Accounting Policies - Share Repurchase Programs (Details) (USD $)	12 Months Ended
	Dec. 31, 2014 repurchase_request	Dec. 31, 2013 repurchase_request	Dec. 31, 2012 repurchase_request
Class of Stock [Line Items]
Number of shares	551,664
ARCT III and ARCT IV Share Repurchase Programs [Member]
Class of Stock [Line Items]
Number of requests	86	11	75
Number of shares	185,700	4,956	180,744
Average price per share	$ 10.47	$ 24.98	$ 10.07