Financial instruments with off-balance sheet risk or concentrations of credit risk (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Guarantor Obligations [Line Items]
Asset	$ 28,134,000	$ 39,045,000
Liability	(7,569,000)	(8,819,000)
Written Credit Derivatives
Guarantor Obligations [Line Items]
Credit Derivative, Collateral	531,900,000	689,500,000
0-5 years	8,652,000	9,882,000
5 years or Greater Expiring Through 2046	2,218,000	550,000
Total Written Credit Default Swaps	10,870,000	10,432,000
Asset	924,000	1,157,000
Liability	0	(348,000)
Net Asset/(Liability)	924,000	809,000
Written Credit Derivatives | Single name (0 - 250)
Guarantor Obligations [Line Items]
0-5 years	0	368,000
5 years or Greater Expiring Through 2046	0	0
Total Written Credit Default Swaps	0	368,000
Asset	0	0
Liability	0	(104,000)
Net Asset/(Liability)	0	(104,000)
Written Credit Derivatives | Single name (251-500)
Guarantor Obligations [Line Items]
0-5 years	8,652,000	9,514,000
5 years or Greater Expiring Through 2046	2,218,000	0
Total Written Credit Default Swaps	10,870,000	9,514,000
Asset	924,000	1,136,000
Liability	0	0
Net Asset/(Liability)	924,000	1,136,000
Written Credit Derivatives | Index (0-250)
Guarantor Obligations [Line Items]
0-5 years	0	0
5 years or Greater Expiring Through 2046	0	550,000
Total Written Credit Default Swaps	0	550,000
Asset	0	21,000
Liability	0	(244,000)
Net Asset/(Liability)	0	(223,000)
Credit risk exposure
Guarantor Obligations [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	16,400,000	19,000,000
Estimate of Fair Value Measurement [Member] | Written Credit Derivatives
Guarantor Obligations [Line Items]
Credit Derivative, Collateral	$ 4,000,000	$ 2,600,000