Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance of other assets and other liabilities	$ 34,051,000	$ 13,171,000	$ 6,923,000	$ 2,244,000
Included in earnings of other assets and other liabilities	84,864,000	13,565,000	157,381,000	56,809,000
Transfer in and/or out of Level 3 of other assets and other liabilities	(48,570,000)	(18,799,000)	(93,959,000)	(51,116,000)
Ending Balance of other assets and other liabilities	70,345,000	7,937,000	70,345,000	7,937,000
Ending Balance	1,193,182		1,193,182
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	63,357,000	5,926,000	70,187,000	7,932,000
Privately issued residential [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,068,392,000	1,306,202,000	1,136,256,000	1,435,561,000
Included in earnings	(5,672,000)	(9,642,000)	(27,976,000)	(41,713,000)
Included in other comprehensive income	39,963,000	(4,030,000)	87,646,000	95,526,000
Settlements	(47,615,000)	(77,452,000)	(140,858,000)	(274,296,000)
Transfers in and/or out of Level 3
Ending Balance	1,055,068,000	1,215,078,000	1,055,068,000	1,215,078,000
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	(5,672,000)	(9,642,000)	(27,976,000)	(41,713,000)
Privately issued commercial [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	12,127,000	17,233,000	15,029,000	22,407,000
Included in other comprehensive income	640,000	1,581,000	1,751,000	99,000
Settlements	(2,199,000)	(1,985,000)	(6,212,000)	(5,677,000)
Transfers in and/or out of Level 3
Ending Balance	10,568,000	16,829,000	10,568,000	16,829,000
Collateralized debt obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	55,098,000	61,601,000	52,500,000	110,756,000
Included in other comprehensive income	3,297,000	(9,482,000)	7,008,000	(2,276,000)
Purchases				50,790,000
Sales				(105,643,000)
Settlements	(1,194,000)	(765,000)	(2,307,000)	(2,273,000)
Transfers in and/or out of Level 3
Ending Balance	$ 57,201,000	$ 51,354,000	$ 57,201,000	$ 51,354,000