Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 14,488,289	$ 11,823,968
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]	2,557,200	1,979,200
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		$ 842,100	$ 1,266,900

[1] Includes $842.1 million and $1,266.9 million of collateralized loan obligations as of September 30, 2019 and December 31, 2018, respectively.