Commitments and contingencies (Future minimum lease payments) (Details) - Office Space And Office Equipment - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Future minimum lease commitments are as follows:
2017	$ 11,878
2018	12,214
2019	11,958
2020	11,059
2021	10,316
2022 and thereafter	46,418
Total	103,843
Total rent expense with respect to operating leases:
Rent expense	$ 9,761	$ 10,143	$ 10,540