Amortized Cost or Cost and Fair Value of Available For Sale Securities (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities	$ 15,065,785		$ 14,802,890
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	1,151,100	[1]	704,400	[1]
Other asset-backed 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	$ 770,000		$ 237,900

[1] Includes $770.0 million and $237.9 million of collateralized loan obligations as of June 30, 2014 and December 31, 2013, respectively.