Organization and Significant Accounting Policies - Narrative (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Significant Accounting Policies [Line Items]
Allowance for loan losses	$ 0	$ 0
Interest Rate Swaps
Schedule Of Significant Accounting Policies [Line Items]
Cash on margin with counterparty to interest rate swaps	$ 1,600,000,000	$ 371,800,000

The amount of cash or securities placed with a broker or counterparty as security for a trading or derivatives securities position which was partially obtained with funds provided by the broker dealer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.8,17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_MarginDepositAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant