Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 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	$ 14,670,207		$ 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	1,605,900	[1]	1,519,900	[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	$ 907,100		$ 900,700

[1] Includes $907.1 million and $900.7 million of collateralized loan obligations as of March 31, 2015 and December 31, 2014, respectively.