Quarterly financial data (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly financial data (unaudited) [Abstract]
Revenue	$ 147,975	$ 137,954	$ 110,325	$ 53,884	$ 98,107	$ 89,102	$ 82,436	$ 43,100	$ 450,138	$ 312,745	$ 281,888
Income (loss) from operations	44,620	31,032	7,862	(25,458)	23,293	18,147	14,712	(13,350)	58,056	42,802	29,967
Net income (loss)	$ 30,116	$ 21,832	$ 5,029	$ (20,236)	$ 15,222	$ 13,491	$ 10,484	$ (8,062)	$ 36,741	$ 31,135	$ 23,524
Net income (loss) per share, basic (in dollars per share)	$ 0.80	$ 0.58	$ 0.13	$ (0.55)	$ 0.42	$ 0.37	$ 0.29	$ (0.22)	$ 0.98	$ 0.86	$ 0.65
Net income (loss) per share, diluted (in dollars per share)	$ 0.66	$ 0.49	$ 0.13	$ (0.55)	$ 0.41	$ 0.36	$ 0.29	$ (0.22)	$ 0.88	$ 0.85	$ 0.65