Shareholders’ equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	9 Months Ended	38 Months Ended		9 Months Ended
	Aug. 31, 2011	Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Series A Convertible Preferred Stock [Member]	Sep. 30, 2013 Series A Convertible Preferred Stock [Member]	Sep. 30, 2014 Treasury Stock [Member]	Sep. 30, 2013 Treasury Stock [Member]	Sep. 30, 2014 Common Stock [Member]	Sep. 30, 2013 Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized (in shares)		195,000,000	195,000,000	195,000,000
Stock repurchase program, authorized amount	$ 60,000
Repurchase of common stock (in shares)			4,786,521
Repurchase of common stock, aggregate purchase price			49,924
Stock repurchase program, remaining authorized repurchase amount		$ 10,076
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Beginning balance					79,620	80,283	4,786,521	4,786,521	88,710,277	88,548,637
Conversion of convertible preferred stock (in shares)					(950)	(549)	0	0	95,039	54,825
Issuance for stock compensation plans (in shares)					0	0	0	0	132,554	79,469
Shares, Ending balance					78,670	79,734	4,786,521	4,786,521	88,937,870	88,682,931