Equity awards (Details)	6 Months Ended
	Jun. 30, 2016 USD ($) Plan $ / shares shares	Jul. 14, 2016 USD ($)
Equity awards [Abstract]
Number of stock-based employee compensation plans | Plan	2
Aggregate intrinsic value [Abstract]
Stock Repurchase Program, Authorized Amount | $		$ 50,000,000
Stock Options Member [Member]
Aggregate Intrinsic Value [Abstract]
Outstanding, beginning of period | $	$ 52,119,607
Outstanding, end of period | $	10,318,273
Exercisable, end of period | $	$ 9,850,228
2006 Plan [Member] | Restricted Stock Units (RSUs) [Member]
Restricted stock unit award activity [Roll Forward]
Outstanding, beginning of period (in shares)	889,004
Granted (in shares)	470,911
Vested (in shares)	(404,585)
Forfeited (in shares)	(26,103)
Outstanding, end of period (in shares)	929,227
Weighted-Average Grant Price [Roll Forward]
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 26.86
Granted (in dollars per share) | $ / shares	34.53
Vested (in dollars per share) | $ / shares	24.77
Forfeited (in dollars per share) | $ / shares	30.95
Outstanding, end of period (in dollars per share) | $ / shares	$ 31.53
Aggregate intrinsic value [Abstract]
Outstanding, beginning of period | $	$ 35,569,048
Outstanding, end of period | $	$ 26,129,863
2006 Plan [Member] | Stock Options Member [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	2,964,237
Granted (in shares)	391,158
Exercised (in shares)	(700,605)
Forfeited (in shares)	(24,467)
Outstanding, end of period (in shares)	2,630,323
Exercisable, end of period (in shares)	1,479,516
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	19.32
Forfeited (in dollars per share) | $ / shares	26.76
Outstanding, end of period (in dollars per share) | $ / shares	25.25
Exercisable, end of period (in dollars per share) | $ / shares	$ 21.55
Aggregate intrinsic value [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	3,800,000
2004 Plan [Member] | Stock Options Member [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares)	29,699
Granted (in shares)	0
Exercised (in shares)	(29,699)
Forfeited (in shares)	0
Outstanding, end of period (in shares)	0
Exercisable, end of period (in 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