Shareholders' equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	9 Months Ended	14 Months Ended
	Aug. 31, 2011	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common stock (in shares)		400,000
Common stock, shares authorized (in shares)		195,000,000	195,000,000	195,000,000
Stock repurchase program, Number of shares authorized to be repurchased (in shares)	$ 60,000
Number of shares Treasury stock acquired (in shares)			4,786,521
Treasury stock acquired, Aggregate purchase price			49,924
Stock repurchase program, Remaining authorized repurchase amount (in shares)		$ 10,076
Series A Convertible Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Beginning balance		80,718
Repurchase of common stock (in shares)		0
Conversion of convertible preferred stock (in shares)		(176)
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		0
Shares, Ending balance		80,542	80,542
Treasury Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Beginning balance		4,386,521
Repurchase of common stock (in shares)		400,000
Conversion of convertible preferred stock (in shares)		0
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		0
Shares, Ending balance		4,786,521	4,786,521
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Beginning balance		88,844,327
Repurchase of common stock (in shares)		(400,000)
Conversion of convertible preferred stock (in shares)		17,587
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		24,531
Shares, Ending balance		88,486,445	88,486,445