Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 15,579,894	$ 14,211,745
Other 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]	2,321,700	2,549,600
Collateralized loan obligations | Other 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		$ 760,500	$ 834,200

[1] Includes $760.5 million and $834.2 million of collateralized loan obligations as of September 30, 2020 and December 31, 2019, respectively.