Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 12,066,223	$ 12,721,399
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]	1,659,500	1,604,600
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		$ 1,071,800	$ 1,101,300

[1] Includes $1,071.8 million and $1,101.3 million of collateralized loan obligations as of March 31, 2018 and December 31, 2017, respectively.