Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Privately Issued Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,012,886,000	$ 1,136,256,000
Total gains (losses) realized/unrealized:
Included in earnings	(9,800,000)	(7,676,000)
Included in other comprehensive income	27,759,000	25,070,000
Settlements	(45,156,000)	(42,955,000)
Ending Balance	985,689,000	1,110,695,000
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	(9,800,000)	(7,676,000)
Privately Issued Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	11,000,000	15,029,000
Total gains (losses) realized/unrealized:
Included in other comprehensive income	(1,378,000)	348,000
Settlements	(2,064,000)	(1,942,000)
Ending Balance	7,558,000	13,435,000
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	61,869,000	52,500,000
Total gains (losses) realized/unrealized:
Included in other comprehensive income	740,000	6,423,000
Settlements	(891,000)	(739,000)
Ending Balance	61,718,000	58,184,000
Other assets and other liabilities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	47,859,000	6,923,000
Total gains (losses) realized/unrealized:
Included in earnings	43,312,000	27,031,000
Transfers in and/or out of Level 3	(55,052,000)	(13,519,000)
Ending Balance	36,119,000	20,435,000
Changes in unrealized gains (losses) included in earnings related to assets still held at end of period	$ 31,398,000	$ 19,506,000