Fair value of financial instruments - Fair value inputs, quantitative information (Details) (Loans receivable [Member], Residential mortgage [Member], Fair value, inputs, level 3 [Member], USD $)	3 Months Ended
	Mar. 31, 2013
Minimum [Member]
Fair value inputs, assets, quantitative information [Line Items]
Discount rate	15.00%
Gross monthly rental rates	$ 690
Home pricing index	(1.40%)
Loan resolution probabilities - modification	0.00%
Loan resolution probabilities - rental	0.00%
Loan resolution probabilities - short sale	0.00%
Loan resolution probabilities - liquidation	0.00%
Loan resolution timelines	1 month
Maximum [Member]
Fair value inputs, assets, quantitative information [Line Items]
Discount rate	15.00%
Gross monthly rental rates	$ 2,800
Home pricing index	1.80%
Loan resolution probabilities - modification	22.30%
Loan resolution probabilities - rental	100.00%
Loan resolution probabilities - short sale	60.00%
Loan resolution probabilities - liquidation	98.80%
Loan resolution timelines	60 months