Changes to Accumulated Other Comprehensive Income (Loss) by Component (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			$ 138,332
Total other comprehensive (loss) income	(24,600)	(13,644)	(20,049)	41,363
Balance at End of Period	118,283		118,283
Net unrealized gains on available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	183,603		186,580
Other comprehensive (loss) income before reclassifications	(38,012)		(28,982)
Amounts reclassified from other accumulated comprehensive income	(242)		(12,249)
Total other comprehensive (loss) income	(38,254)		(41,231)
Balance at End of Period	145,349		145,349
Net unrealized losses on interest rate agreements accounted for as cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	(40,720)		(48,248)
Other comprehensive (loss) income before reclassifications	13,585		21,025
Amounts reclassified from other accumulated comprehensive income	69		157
Total other comprehensive (loss) income	13,654		21,182
Balance at End of Period	$ (27,066)		$ (27,066)