Reclassifications out of Accumulated Other Comprehensive Income (Loss) (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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other than temporary impairment	$ (1,642)	[1]	$ (303)	[1]	$ (1,666)	[1]	$ (635)	[1]
Realized gains, net	556		6,995		12,823		20,615
Total interest expense	21,463		28,881		39,817		59,536
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other than temporary impairment	(133)				(124)
Realized gains, net	(242)				(12,249)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Investment Gain (Loss) | Gain on sale of AFS securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain (loss) on sale of AFS securities	(109)				(12,125)
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized losses on interest rate agreements accounted for as cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total interest expense	69				157
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized losses on interest rate agreements accounted for as cash flow hedges | Hedge ineffectiveness
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total interest expense	$ 69				$ 157

[1] For the three months ended June 30, 2013, other-than-temporary impairments were $1,642, none of which were recognized in Accumulated Other Comprehensive Income. For the three months ended June 30, 2012, other-than-temporary impairments were $411, of which $108 were recognized in Accumulated Other Comprehensive Income. For the six months ended June 30, 2013, other-than-temporary impairments were $1,666, none of which were recognized in Accumulated Other Comprehensive Income. For the six months ended June 30, 2012, other-than-temporary impairments were $892, of which $257 were recognized in Accumulated Other Comprehensive Income.