Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Privately issued residential [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,136,256,000	$ 1,435,561,000	$ 2,064,904,000
Included in earnings	(42,467,000)	(64,919,000)	(63,503,000)
Included in other comprehensive income	110,309,000	6,489,000	135,434,000
Settlements	(191,212,000)	(240,875,000)	(346,026,000)
Transfers in and/or out of Level 3			(355,248,000)
Ending Balance	1,012,886,000	1,136,256,000	1,435,561,000
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	(42,467,000)	(64,919,000)	(63,503,000)
Privately issued commercial [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	15,029,000	22,407,000	25,166,000
Included in other comprehensive income	4,283,000	327,000	5,462,000
Settlements	(8,312,000)	(7,705,000)	(8,221,000)
Ending Balance	11,000,000	15,029,000	22,407,000
Collateralized debt obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	52,500,000	110,756,000	115,346,000
Included in earnings		19,231,000	(5,703,000)
Included in other comprehensive income	12,214,000	(272,000)	2,887,000
Settlements	(2,845,000)	(3,131,000)	(1,774,000)
Ending Balance	61,869,000	52,500,000	110,756,000
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period			(5,703,000)
Purchases		50,790,000
Sales		(124,874,000)
Other debt securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance			420,000
Included in other comprehensive income			35,000
Transfers in and/or out of Level 3			(455,000)
Other assets and other liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	6,923,000	2,244,000	(80,000)
Included in earnings	212,281,000	67,163,000	95,661,000
Transfers in and/or out of Level 3	(171,345,000)	(62,484,000)	(93,337,000)
Ending Balance	47,859,000	6,923,000	2,244,000
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	$ 47,859,000	$ 6,902,000	$ 2,153,000