Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Out Of Accumulated Other Comprehensive Loss [LineItems]
Gains (losses) on securities transactions, net		$ 6	$ 157	$ (92)	$ 2,416	$ 643	$ 103	$ 7	$ (8)	$ 2,487	$ 745	$ 14,678
Interest and dividends on investment										728	661	461
Interest expense		(37,548)	(40,730)	(39,577)	(38,899)	(42,296)	(40,791)	(39,745)	(39,014)	(156,754)	(161,846)	(168,377)
Amortization of prior service cost										0	0	(7,104)
Amortization of net actuarial loss										17,177	(28,749)
Income (loss) before income taxes		(6,316)	46,133	44,465	42,613	32,962	38,336	41,271	34,665	126,895	147,234	178,940
Amounts Reclassified from Accumulated Other Comprehensive Loss, tax effect		10,987	(10,179)	(12,474)	(12,272)	$ (7,827)	$ (10,654)	$ (11,751)	$ (830)	(23,938)	(31,062)	(46,979)
Net Income Available to Common Shareholders		$ 2,875	$ 33,937	$ 31,991	$ 30,341					99,144	116,172	131,961
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Out Of Accumulated Other Comprehensive Loss [LineItems]
Interest expense										(7,075)	(6,663)	$ (6,898)
Net Income Available to Common Shareholders										(2,836)	(3,379)
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member] | Unrealized Gains and Losses on AFS Securities [Member]
Reclassification Out Of Accumulated Other Comprehensive Loss [LineItems]
Gains (losses) on securities transactions, net										2,487	745
Amounts Reclassified from Accumulated Other Comprehensive Loss, tax effect										(1,041)	(312)
Net Income Available to Common Shareholders										1,446	433
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member] | Non-Credit Impairment Losses on Securities [Member]
Reclassification Out Of Accumulated Other Comprehensive Loss [LineItems]
Interest and dividends on investment										728	661
Amounts Reclassified from Accumulated Other Comprehensive Loss, tax effect										(304)	(278)
Net Income Available to Common Shareholders										424	383
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member] | Unrealized Losses on Derivatives (Cash Flow Hedges) Before Tax [Member]
Reclassification Out Of Accumulated Other Comprehensive Loss [LineItems]
Interest expense										(7,075)	(6,663)
Amounts Reclassified from Accumulated Other Comprehensive Loss, tax effect										2,948	2,777
Net Income Available to Common Shareholders										(4,127)	(3,886)
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member] | Defined Benefit Pension Plan [Member]
Reclassification Out Of Accumulated Other Comprehensive Loss [LineItems]
Amortization of prior service cost	[1]									(206)	(305)
Amortization of net actuarial loss	[1]									(790)	(226)
Income (loss) before income taxes										(996)	(531)
Amounts Reclassified from Accumulated Other Comprehensive Loss, tax effect										417	222
Net Income Available to Common Shareholders										$ (579)	$ (309)

[1] These accumulated other comprehensive loss components are included in the computation of net periodic pension cost. See Note 12 for details.