Quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly financial data (unaudited) [Abstract]
Revenue	$ 168,104	$ 164,940	$ 126,112	$ 63,633	$ 147,975	$ 137,954	$ 110,325	$ 53,884	$ 522,789	$ 450,138	$ 312,745
Income (loss) from operations	49,892	53,005	21,452	(28,310)	44,620	31,032	7,862	(25,458)	96,039	58,056	42,802
Net income (loss)	$ 33,347	$ 36,942	$ 14,100	$ (21,519)	$ 30,116	$ 21,832	$ 5,029	$ (20,236)	$ 62,870	$ 36,741	$ 31,135
Net income (loss) per share, basic (in dollars per share)	$ 0.85	$ 0.95	$ 0.37	$ (0.57)	$ 0.80	$ 0.58	$ 0.13	$ (0.55)	$ 1.63	$ 0.98	$ 0.86
Net income (loss) per share, diluted (in dollars per share)	$ 0.71	$ 0.79	$ 0.32	$ (0.57)	$ 0.66	$ 0.49	$ 0.13	$ (0.55)	$ 1.41	$ 0.88	$ 0.85