Amortized Cost or Cost and Fair Value of Available For Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 12,983,213	$ 13,605,963
Other asset-backed securities | Debt Securities | Mortgage and asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities	[1]	1,489,900	1,566,500
Other asset-backed securities | Debt Securities | Mortgage and asset backed securities | Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 903,800	$ 946,700

[1] Includes $903.8 million and $946.7 million of collateralized loan obligations as of December 31, 2016 and 2015, respectively.