Equity awards (Details)	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares shares
Stock Options Member [Member]
Aggregate Intrinsic Value [Abstract]
Outstanding, beginning of period | $	$ 52,119,607
Outstanding, end of period | $	38,049,150
Exercisable, end of period | $	$ 29,719,065
2006 Plan [Member] | Restricted Stock Units (RSUs) [Member]
Restricted stock unit award activity [Roll Forward]
Outstanding, beginning of period (in shares) | shares	889,004
Granted (in shares) | shares	427,592
Vested (in shares) | shares	(353,117)
Forfeited (in shares) | shares	(9,599)
Outstanding, end of period (in shares) | shares	953,880
Weighted-Average Grant Price [Roll Forward]
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 26.86
Granted (in dollars per share) | $ / shares	33.83
Vested (in dollars per share) | $ / shares	24.70
Forfeited (in dollars per share) | $ / shares	30.06
Outstanding, end of period (in dollars per share) | $ / shares	$ 30.75
Aggregate intrinsic value [Abstract]
Outstanding, beginning of period | $	$ 35,569,048
Outstanding, end of period | $	$ 34,673,539
2006 Plan [Member] | Stock Options Member [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares) | shares	2,964,237
Granted (in shares) | shares	391,158
Exercised (in shares) | shares	(178,124)
Forfeited (in shares) | shares	(12,742)
Outstanding, end of period (in shares) | shares	3,164,529
Exercisable, end of period (in shares) | shares	1,951,044
Weighted-Average Exercise Price [Roll Forward]
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 22.73
Granted (in dollars per share) | $ / shares	33.83
Exercised (in dollars per share) | $ / shares	18.47
Forfeited (in dollars per share) | $ / shares	25.48
Outstanding, end of period (in dollars per share) | $ / shares	24.33
Exercisable, end of period (in dollars per share) | $ / shares	$ 21.12
2004 Plan [Member] | Stock Options Member [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares) | shares	29,699
Granted (in shares) | shares	0
Exercised (in shares) | shares	(29,699)
Forfeited (in shares) | shares	0
Outstanding, end of period (in shares) | shares	0
Exercisable, end of period (in shares) | shares	0
Weighted-Average Exercise Price [Roll Forward]
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 10.28
Granted (in dollars per share) | $ / shares	0
Exercised (in dollars per share) | $ / shares	10.28
Forfeited (in dollars per share) | $ / shares	0
Outstanding, end of period (in dollars per share) | $ / shares	0
Exercisable, end of period (in dollars per share) | $ / shares	$ 0