Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Loss after Tax (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Accumulated Other Comprehensive Income (Loss), Beginning Balance	$ (38,252)
Total other comprehensive (loss) gain	(4,243)	12,657	11,532
Total Accumulated Other Comprehensive Income (Loss), Ending Balance	(42,495)	(38,252)
Unrealized Gains and Losses on AFS Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Accumulated Other Comprehensive Income (Loss), Beginning Balance	(20,855)	906	(3,231)
Other comprehensive income (loss) before reclassifications	19,398	(13,239)	5,682
Amounts reclassified from other comprehensive income (loss)	(433)	(8,522)	(1,545)
Total other comprehensive (loss) gain	18,965	(21,761)	4,137
Total Accumulated Other Comprehensive Income (Loss), Ending Balance	(1,890)	(20,855)	906
Non-Credit Impairment Losses on Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Accumulated Other Comprehensive Income (Loss), Beginning Balance	(806)	(4,175)	(16,566)
Other comprehensive income (loss) before reclassifications	1,334	3,637	9,942
Amounts reclassified from other comprehensive income (loss)	(383)	(268)	2,449
Total other comprehensive (loss) gain	951	3,369	12,391
Total Accumulated Other Comprehensive Income (Loss), Ending Balance	145	(806)	(4,175)
Unrealized Gains and Losses on Derivatives [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Accumulated Other Comprehensive Income (Loss), Beginning Balance	(6,271)	(12,676)	(13,085)
Other comprehensive income (loss) before reclassifications	(12,147)	2,400	(3,351)
Amounts reclassified from other comprehensive income (loss)	3,886	4,005	3,760
Total other comprehensive (loss) gain	(8,261)	6,405	409
Total Accumulated Other Comprehensive Income (Loss), Ending Balance	(14,532)	(6,271)	(12,676)
Defined Benefit Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Accumulated Other Comprehensive Income (Loss), Beginning Balance	(10,320)	(34,964)	(29,559)
Other comprehensive income (loss) before reclassifications	(16,207)	21,577	(6,839)
Amounts reclassified from other comprehensive income (loss)	309	3,067	1,434
Total other comprehensive (loss) gain	(15,898)	24,644	(5,405)
Total Accumulated Other Comprehensive Income (Loss), Ending Balance	(26,218)	(10,320)	(34,964)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total Accumulated Other Comprehensive Income (Loss), Beginning Balance	(38,252)	(50,909)	(62,441)
Other comprehensive income (loss) before reclassifications	(7,622)	14,375	5,434
Amounts reclassified from other comprehensive income (loss)	3,379	(1,718)	6,098
Total other comprehensive (loss) gain	(4,243)	12,657	11,532
Total Accumulated Other Comprehensive Income (Loss), Ending Balance	$ (42,495)	$ (38,252)	$ (50,909)