Amortized Cost and Fair Value of Available For Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 14,945,479	$ 15,618,470
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities	[1]	2,412,300	2,351,600
Other asset-backed securities | Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 959,700	$ 878,400

[1] Includes $959.7 million and $878.4 million of collateralized loan obligations as of March 31, 2021 and December 31, 2020, respectively.