Equity awards (Details) - 6 months ended Jun. 30, 2015 - USD ($)	Total
Stock Options Member [Member]
Aggregate Intrinsic Value [Abstract]
Outstanding, beginning of period	$ 29,181,534
Outstanding, end of period	40,371,293
Exercisable, end of period	$ 28,238,161
2006 Plan [Member] | Restricted Stock Units (RSUs) [Member]
Restricted stock unit award activity [Roll Forward]
Outstanding, beginning of period (in shares)	927,356
Granted (in shares)	457,161
Vested (in shares)	(393,811)
Forfeited (in shares)	(29,017)
Outstanding, end of period (in shares)	961,689
Weighted-Average Grant Price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 22.44
Granted (in dollars per share)	29.44
Vested (in dollars per share)	20.73
Forfeited (in dollars per share)	25.19
Outstanding, end of period (in dollars per share)	$ 26.38
Aggregate intrinsic value [Abstract]
Outstanding, beginning of period	$ 25,251,904
Outstanding, end of period	$ 31,687,654
2006 Plan [Member] | Stock Options Member [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	3,837,993
Granted (in shares)	685,841
Exercised (in shares)	(734,596)
Forfeited (in shares)	(80,257)
Outstanding, end of period (in shares)	3,708,981
Exercisable, end of period (in shares)	2,014,030
Weighted-Average Exercise Price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 20.04
Granted (in dollars per share)	29.00
Exercised (in dollars per share)	16.87
Forfeited (in dollars per share)	23.17
Outstanding, end of period (in dollars per share)	22.25
Exercisable, end of period (in dollars per share)	$ 19.26
2004 Plan [Member] | Stock Options Member [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	43,156
Granted (in shares)	0
Exercised (in shares)	(13,457)
Forfeited (in shares)	0
Outstanding, end of period (in shares)	29,699
Exercisable, end of period (in shares)	29,699
Weighted-Average Exercise Price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 10.28
Granted (in dollars per share)	0
Exercised (in dollars per share)	31.33
Forfeited (in dollars per share)	0
Outstanding, end of period (in dollars per share)	10.28
Exercisable, end of period (in dollars per share)	$ 10.28