Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 12,721,399	$ 12,983,213
Other asset-backed securities | Debt 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]	1,604,600	1,489,900
Other asset-backed securities | Debt Securities | Mortgage and asset backed securities | Collateralized loan obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 1,101,300	$ 903,800

[1] Includes $1,101.3 million and $903.8 million of collateralized loan obligations as of December 31, 2017 and 2016, respectively.