Change in Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 19,532		$ 16,763	$ 35,132	$ 31,816
Income taxes	(6,807)		(5,855)	(5,496)	(11,168)
Income taxes		6,700
Net income allocable to common stockholders	12,725		10,299	29,636	19,347
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income allocable to common stockholders	166		562	525	1,581
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized (Losses) Gains on Investment Securities Available-for-Sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	365		906	943	2,550
Income taxes	(139)		(344)	(358)	(969)
Net of tax	226		562	585	1,581
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Losses on Defined Benefit Pension Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service costs	919			919
Transition obligation	245			245
Actuarial losses	(1,068)			(1,068)
Total before tax	96			96
Income taxes	(36)			(36)
Net income allocable to common stockholders	$ 60			$ 60