Amortized Cost or Cost and Fair Value of Available For Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 13,681,224	$ 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]	1,568,400	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		$ 947,200	$ 946,700

[1] Includes $947.2 million and $946.7 million of collateralized loan obligations as of June 30, 2016 and December 31, 2015, respectively.