Commitments and contingencies Textual (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Other Commitments [Line Items]
Operating Leases, Rent Expense	$ 2,100,000	$ 1,900,000	$ 6,100,000	$ 5,800,000
Cash Reserve Deposit Required and Made	42,000,000		$ 42,000,000		$ 33,000,000
Minimum [Member]
Other Commitments [Line Items]
Operating Leases, Lease Renewal Period Option			1 year
Maximum [Member]
Other Commitments [Line Items]
Operating Leases, Lease Renewal Period Option			10 years
Affiliated Entity [Member]
Other Commitments [Line Items]
Operating Leases, Rent Expense	$ 0	$ 92,000	$ 0	$ 216,000