Shareholders' equity (Details) - USD ($)	6 Months Ended		15 Months Ended	60 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Dec. 31, 2011	Dec. 31, 2008
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	195,000,000		195,000,000	195,000,000
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01	$ 0.01
Common stock, shares issued	94,262,108		94,262,108	94,224,571
Common stock, shares outstanding	87,075,587		87,075,587	87,038,050
Stock repurchase program, authorized amount				$ 130,000,000	$ 60,000,000
Repurchase of common stock (in shares)			0	7,186,521
Repurchase of common stock, aggregate purchase price				$ 86,276,000
Stock repurchase program, remaining authorized repurchase amount	$ 43,724,000		$ 43,724,000
Preferred stock, shares authorized	5,000,000		5,000,000	5,000,000
Preferred stock par value (in dollars per share)	$ 0.01		$ 0.01	$ 0.01
Preferred shares issued						160,000
Number of shares issued for each share of preferred stock	100		100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Repurchase of common stock (in shares)			0	7,186,521
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Repurchase of common stock (in shares)	0	0
Preferred stock par value (in dollars per share)	$ 0.01		$ 0.01
Preferred shares outstanding	76,378		76,378	76,539
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Beginning balance (in shares)	76,539	78,141
Repurchase of common stock (in shares)	0	0
Conversion of convertible preferred stock (in shares)	(161)	(1,014)
Issuance for stock compensation plans (in shares)	0	0
Shares, Ending balance (in shares)	76,378	77,127	76,378	76,539
Treasury Stock [Member]
Class of Stock [Line Items]
Repurchase of common stock (in shares)	0	2,400,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Beginning balance (in shares)	7,186,521	4,786,521
Repurchase of common stock (in shares)	0	2,400,000
Conversion of convertible preferred stock (in shares)	0	0
Issuance for stock compensation plans (in shares)	0	0
Shares, Ending balance (in shares)	7,186,521	7,186,521	7,186,521	7,186,521
Common Stock [Member]
Class of Stock [Line Items]
Repurchase of common stock (in shares)	0	(2,400,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Beginning balance (in shares)	87,038,050	89,064,582
Repurchase of common stock (in shares)	0	(2,400,000)
Conversion of convertible preferred stock (in shares)	16,109	101,327
Issuance for stock compensation plans (in shares)	21,428	135,043
Shares, Ending balance (in shares)	87,075,587	86,900,952	87,075,587	87,038,050