Financial instruments - Schedule of Gross Amounts and Offsetting Amounts of Reverse Repurchase Agreements, Repurchase Agreements, Securities Borrowed and Securities Lending Transactions (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Assets	$ 4,049,058	$ 1,525,881
Gross Amounts Offset in the Statement of Financial Position	3,862,697	1,160,239
Net Amounts of Assets Presented on the Balance Sheet	186,361	365,642
Financial Instruments	(186,361)	(365,642)
Cash Collateral Received
Net Amount
Gross Amounts of Recognized Liabilities	4,582,434	1,743,017
Gross Amounts Offset in the Statement of Financial Position	3,862,697	1,160,239
Net Amounts of Liabilities Presented on the Balance Sheet	719,737	582,778
Financial Instruments	(719,737)	(582,778)
Cash Collateral Pledged
Net Amount
Repurchase agreements [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Liabilities	4,406,833	1,552,630
Gross Amounts Offset in the Statement of Financial Position	3,862,697	1,160,239
Net Amounts of Liabilities Presented on the Balance Sheet	544,136	392,391
Financial Instruments	(544,136)	(392,391)
Cash Collateral Pledged
Net Amount
Securities lending [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Liabilities	175,601	190,387
Net Amounts of Liabilities Presented on the Balance Sheet	175,601	190,387
Financial Instruments	(175,601)	(190,387)
Cash Collateral Pledged
Net Amount
Reverse repurchase agreement [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Assets	3,862,697	1,160,239
Gross Amounts Offset in the Statement of Financial Position	3,862,697	1,160,239
Cash Collateral Received
Net Amount
Securities borrowing [Member]
Securities Financing Transaction [Line Items]
Gross Amounts of Recognized Assets	186,361	365,642
Net Amounts of Assets Presented on the Balance Sheet	186,361	365,642
Financial Instruments	(186,361)	(365,642)
Cash Collateral Received
Net Amount

Financial instruments - Schedule of Gross Amounts and Offsetting Amounts of Reverse Repurchase Agreements, Repurchase Agreements, Securities Borrowed and Securities Lending Transactions (Detail) (USD $)
In Thousands, unless otherwise specified

Mar. 31, 2013

Dec. 31, 2012

Securities Financing Transaction [Line Items]

Gross Amounts of Recognized Assets

$ 4,049,058

$ 1,525,881

Gross Amounts Offset in the Statement of Financial Position

3,862,697

1,160,239

Net Amounts of Assets Presented on the Balance Sheet

186,361

365,642