Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	$ 14,766,235		$ 14,802,890
Other asset-backed securities | Mortgage and asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	1,322,800	[1]	704,400	[1]
Other asset-backed securities | Collateralized loan obligations | Mortgage and asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	$ 887,500		$ 237,900

[1] Includes $887.5 million and $237.9 million of collateralized loan obligations as of September 30, 2014 and December 31, 2013, respectively.