Commitments and contingencies Textual (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Other Commitments [Line Items]
Operating Leases, Rent Expense	$ 2.5	$ 2.0
Cash Reserve Deposit Required and Made	$ 0.0		$ 35.8
Minimum [Member]
Other Commitments [Line Items]
Operating Leases, Lease Renewal Period Option	5 years
Maximum [Member]
Other Commitments [Line Items]
Operating Leases, Lease Renewal Period Option	10 years