Common Stock (Details) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended		30 Months Ended
	Sep. 26, 2011	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Dec. 31, 2012
Stockholders' Equity Note [Abstract]
Common stock, outstanding		67,077,160		67,077,160		67,077,160	66,964,162
Common stock, issued		75,398	57,626	215,197	418,640
Stock repurchase, authorized amount (in US dollars)	$ 100,000,000
Number of shares repurchased				100,000		2,200,000
Stock repurchased (in US dollars)				4,998,000	21,189,000	56,400,000
Remaining authorized repurchase amount (in US dollars)				$ 43,600,000