Securities - Securities Available for Sale in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	$ 493,281	$ 114,500
Unrealized Loss, Less Than 12 Months	(30,642)	(710)
Fair Value, 12 Months or More	8,704
Unrealized Loss, 12 Months or More	(257)
Total Fair Value	501,985	114,500
Total, Unrealized Loss	(30,899)	(710)
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	229,262	42,480
Unrealized Loss, Less Than 12 Months	(20,506)	(562)
Fair Value, 12 Months or More
Unrealized Loss, 12 Months or More
Total Fair Value	229,262	42,480
Total, Unrealized Loss	(20,506)	(562)
Mortgage-Related Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less Than 12 Months	264,019	72,020
Unrealized Loss, Less Than 12 Months	(10,136)	(148)
Fair Value, 12 Months or More	8,704
Unrealized Loss, 12 Months or More	(257)
Total Fair Value	272,723	72,020
Total, Unrealized Loss	$ (10,393)	$ (148)