Net income per common share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net income (numerator):
Net income—basic and diluted	$ 33,452	$ 61,110	$ 119,972	$ 129,386
Weighted-average common shares outstanding (denominator):
Basic (in shares)	230,933	239,231	234,561	238,870
Diluted (in shares)	231,706	244,417	235,501	244,385
Net income per common share:
Basic (in dollars per share)	$ 0.14	$ 0.26	$ 0.51	$ 0.54
Diluted (in dollars per share)	$ 0.14	$ 0.25	$ 0.51	$ 0.53
Non-vested restricted stock awards
Weighted-average common shares outstanding (denominator):
Incremental common shares (in shares)	683	419	885	896
Outstanding stock option awards
Weighted-average common shares outstanding (denominator):
Incremental common shares (in shares)	90	101	55	131
Non-vested performance share awards
Weighted-average common shares outstanding (denominator):
Incremental common shares (in shares)	0	4,666	0	4,488