Financial instruments with off-balance sheet risk or concentrations of credit risk (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Guarantor Obligations [Line Items]
Asset	$ 21,130,000	$ 39,045,000
Liability	(11,015,000)	(8,819,000)
Written credit derivatives
Guarantor Obligations [Line Items]
0-5 years	0	9,882,000
5 years or Greater Expiring Through 2046	7,226,000	550,000
Total Written Credit Default Swaps	7,226,000	10,432,000
Asset	205,000	1,157,000
Liability	(1,319,000)	(348,000)
Net Asset/(Liability)	(1,114,000)	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	5,142,000	0
Total Written Credit Default Swaps	5,142,000	368,000
Asset	0	0
Liability	(1,319,000)	(104,000)
Net Asset/(Liability)	(1,319,000)	(104,000)
Written credit derivatives | Single name (251-500)
Guarantor Obligations [Line Items]
0-5 years	0	9,514,000
5 years or Greater Expiring Through 2046	2,084,000	0
Total Written Credit Default Swaps	2,084,000	9,514,000
Asset	205,000	1,136,000
Liability	0	0
Net Asset/(Liability)	205,000	1,136,000
Written credit derivatives | Index (0-250)
Guarantor Obligations [Line Items]
0-5 years		0
5 years or Greater Expiring Through 2046		550,000
Total Written Credit Default Swaps		550,000
Asset		21,000
Liability		(244,000)
Net Asset/(Liability)		(223,000)
Credit risk exposure
Guarantor Obligations [Line Items]
Maximum counterparty credit risk exposure	9,700,000	19,000,000
Written put options
Guarantor Obligations [Line Items]
Option contracts maximum payout	666,900,000	689,500,000
Written put options | Securities sold, not yet purchased
Guarantor Obligations [Line Items]
Options contracts at fair value	$ 4,500,000	$ 2,600,000
Written put options | Minimum
Guarantor Obligations [Line Items]
Option contracts maximum payout term	1 month
Written put options | Maximum
Guarantor Obligations [Line Items]
Option contracts maximum payout term	10 months