Amortized Cost or Cost and Fair Value of Available For Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 13,618,212	$ 13,605,963
Other asset-backed securities | Mortgage and asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities	[1]	1,534,800	1,566,500
Collateralized loan obligations | Other asset-backed securities | Mortgage and asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 913,900	$ 946,700

[1] Includes $913.9 million and $946.7 million of collateralized loan obligations as of March 31, 2016 and December 31, 2015, respectively.