Net income per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Net Income per Share
Net income	$ 16,174	$ 26,100
Weighted-average number of common shares for basic net income per share	96,399,351	96,123,351
Incremental effect of dilutive common stock equivalents:
Unexercised stock options (in shares)	806,311	1,202,004
Unvested restricted stock awards (in shares)	303,866	337,253
Weighted-average number of shares for diluted net income per share	97,509,528	97,662,608
Net income per share - basic (in dollars per share)	$ 0.17	$ 0.27
Net income per share - diluted (in dollars per share)	$ 0.17	$ 0.27
Anti-dilutive unexercised stock options (in shares)	1,463,000	374,000