Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 14,530,255	$ 14,598,641
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]	1,652,700	1,519,900
Collateralized loan obligations | 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		$ 952,400	$ 900,700

[1] Includes $952.4 million and $900.7 million of collateralized loan obligations as of September 30, 2015 and December 31, 2014, respectively.