Commitments and contingencies Textual (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Other Commitments [Line Items]
Operating Leases, Rent Expense	$ 1,900,000	$ 1,938,000	$ 5,800,000	$ 5,700,000
Cash Reserve Deposit Required and Made	37,700,000		37,700,000		21,000,000
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	$ 780,000	$ 759,000	$ 1,500,000	$ 1,504,000