Financial Instruments at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) - USD ($) $ in Thousands		Mar. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 15,558,624	$ 16,061,560
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]	3,087,900	3,001,000
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		$ 1,281,500	$ 1,320,400

[1] Includes $1,281.5 million and $1,320.4 million of collateralized loan obligations as of March 31, 2022 and December 31, 2021, respectively.

[2] Includes $1,281.5 million and $1,320.4 million of collateralized loan obligations as of March 31, 2022 and December 31, 2021, respectively.