Equity awards (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) Plan $ / shares shares
Equity awards [Abstract]
Number of stock-based employee compensation plans | Plan	1
2006 Plan [Member] | Restricted Stock Units (RSUs) [Member]
Restricted stock unit award activity [Roll Forward]
Outstanding, beginning of period (in shares) | shares	875,584
Granted (in shares) | shares	428,810
Vested (in shares) | shares	(403,992)
Forfeited (in shares) | shares	(31,148)
Outstanding, end of period (in shares) | shares	869,254
Weighted-Average Grant Price [Roll Forward]
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 28.94
Granted (in dollars per share) | $ / shares	30.55
Vested (in dollars per share) | $ / shares	20.72
Forfeited (in dollars per share) | $ / shares	28.78
Outstanding, end of period (in dollars per share) | $ / shares	$ 30.20
Aggregate intrinsic value [Abstract]
Outstanding, beginning of period | $	$ 28,754,179
Outstanding, end of period | $	$ 29,476,403
2006 Plan [Member] | Stock Options Member [Member]
Options outstanding [Roll Forward]
Outstanding, beginning of period (in shares) | shares	2,559,331
Granted (in shares) | shares	398,058
Exercised (in shares) | shares	(243,400)
Forfeited (in shares) | shares	(23,796)
Outstanding, end of period (in shares) | shares	2,690,193
Weighted-Average Exercise Price [Roll Forward]
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 22.94
Granted (in dollars per share) | $ / shares	30.59
Exercised (in dollars per share) | $ / shares	19.29
Forfeited (in dollars per share) | $ / shares	27.92
Outstanding, end of period (in dollars per share) | $ / shares	$ 24.35
Aggregate Intrinsic Value [Abstract]
Outstanding, beginning of period | $	$ 25,348,245
Outstanding, end of period | $	$ 25,707,213