Commitments and contingencies Textual (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Other Commitments [Line Items]
Operating Leases, Rent Expense	$ 2,300,000	$ 2,100,000	$ 4,800,000	$ 4,200,000
Operating Leases, Future Minimum Payments Due	0		0
Cash Reserve Deposit Required and Made	$ 35,400,000		$ 35,400,000		$ 35,800,000
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