Capitalized servicing assets - Changes in Capitalized Servicing Assets (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Servicing Assets at Amortized Value [Line Items]
Beginning balance	$ 198,192	$ 160,876	$ 174,302
Originations	34,230	31,533	44,406
Purchases	109	61,642	593
Consolidation of loan securitization trusts (note 19)			(1,843)
Amortization	(59,555)	(55,859)	(56,582)
Amortized value, ending balance	172,976	198,192	160,876
Valuation allowance	(4,500)	(1,800)
Ending balance, net	168,476	196,392	160,876
Commercial Mortgage Loans [Member]
Servicing Assets at Amortized Value [Line Items]
Beginning balance	51,250	42,613	32,896
Originations	19,653	17,613	16,976
Purchases
Consolidation of loan securitization trusts (note 19)
Amortization	(10,925)	(8,976)	(7,259)
Amortized value, ending balance	59,978	51,250	42,613
Valuation allowance
Ending balance, net	59,978	51,250	42,613
Residential Mortgage Loans [Member]
Servicing Assets at Amortized Value [Line Items]
Beginning balance	131,264	92,066	101,155
Originations	14,577	13,920	27,430
Purchases	109	61,642	593
Consolidation of loan securitization trusts (note 19)			(1,843)
Amortization	(41,095)	(36,364)	(35,269)
Amortized value, ending balance	104,855	131,264	92,066
Valuation allowance	(4,500)	(1,800)
Ending balance, net	100,355	129,464	92,066
Small-Balance Commercial Mortgage Loans [Member]
Servicing Assets at Amortized Value [Line Items]
Beginning balance	15,678	26,197	40,251
Originations
Purchases
Consolidation of loan securitization trusts (note 19)
Amortization	(7,535)	(10,519)	(14,054)
Amortized value, ending balance	8,143	15,678	26,197
Valuation allowance
Ending balance, net	$ 8,143	$ 15,678	$ 26,197