Commitments and contingencies Textual (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Other Commitments [Line Items]
Operating Leases, Rent Expense	$ 2,100	$ 2,100	$ 6,300	$ 6,400
Cash Reserve Deposit Required and Made	$ 6,700		$ 6,700		$ 31,800
Minimum [Member]
Other Commitments [Line Items]
Operating Leases, Lease Renewal Period Option			4 months
Maximum [Member]
Other Commitments [Line Items]
Operating Leases, Lease Renewal Period Option			10 years