Financial instruments - Notional and Fair Values of TBAs (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value Of To Be Announced Securities [Abstract]
Notional Amount, Sale of TBAs	$ 345,168	$ 449,065
Notional Amount, Purchase of TBAs	44,749	117,573
Notional Amount, Funding Commitments	264,035	304,390
Notional Amount, Total	308,784	421,963
Fair Value, Sale of TBAs	3,111	3,188
Fair Value, Purchase of TBAs	$ 281	$ 175