Summary of Gross Amounts, Amounts Offset and net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Detail) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2015	Dec. 31, 2014
Repurchase Agreements:
Gross amounts -reverse repurchase agreements			$ 700,000
Amounts offset - reverse repurchase agreement			(600,000)
Netted amounts -reverse repurchase	[1]		100,000
Gross amounts -repurchase agreement		$ 57,459,552	71,961,926
Amounts offset -repurchase agreement			(600,000)
Netted amounts -repurchase agreement		$ 57,459,552	$ 71,361,926	[1]

[1] Derived from the audited consolidated financial statements at December 31, 2014.