Securities - Securities Available for Sale in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	$ 38,170	$ 433,947
Unrealized Loss, Less Than 12 Months	(133)	(32,040)
Fair Value, 12 Months or More	467,705	75,365
Unrealized Loss, 12 Months or More	(16,682)	(8,728)
Total Fair Value	505,875	509,312
Total, Unrealized Loss	(16,815)	(40,768)
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	4,444	193,746
Unrealized Loss, Less Than 12 Months	(46)	(21,360)
Fair Value, 12 Months or More	226,695	28,046
Unrealized Loss, 12 Months or More	(10,358)	(4,210)
Total Fair Value	231,139	221,792
Total, Unrealized Loss	(10,404)	(25,570)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	33,726	240,201
Unrealized Loss, Less Than 12 Months	(87)	(10,680)
Fair Value, 12 Months or More	241,010	47,319
Unrealized Loss, 12 Months or More	(6,324)	(4,518)
Total Fair Value	274,736	287,520
Total, Unrealized Loss	$ (6,411)	$ (15,198)