Changes to Accumulated Other Comprehensive Income (Loss) by Component (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period			$ 138,332
Total other comprehensive (loss) income	(3,515)	45,660	(23,564)	87,023
Balance at End of Period	114,768		114,768
Net unrealized gains on available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period	145,349		186,580
Other comprehensive (loss) income before reclassifications	(633)		(29,615)
Amounts reclassified from other accumulated comprehensive income	(6,962)		(19,211)
Total other comprehensive (loss) income	(7,595)		(48,826)
Balance at End of Period	137,754		137,754
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	(27,066)		(48,248)
Other comprehensive (loss) income before reclassifications	4,018		25,043
Amounts reclassified from other accumulated comprehensive income	62		219
Total other comprehensive (loss) income	4,080		25,262
Balance at End of Period	$ (22,986)		$ (22,986)