Investment Securities Held-to-Maturity - Unrealized Gain (Losses) on Securities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,374,712,000	$ 275,507,000
Gross Unrealized Gains	5,971,000	6,499,000
Gross Unrealized Losses	9,565,000	82,000
Estimated Fair Value	1,371,118,000	281,924,000
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	276,509,000	17,747,000
Gross Unrealized Gains	637,000	19,000
Gross Unrealized Losses	1,756,000	8,000
Estimated Fair Value	275,390,000	17,758,000
U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,098,203,000	257,760,000
Gross Unrealized Gains	5,334,000	6,480,000
Gross Unrealized Losses	7,809,000	74,000
Estimated Fair Value	$ 1,095,728,000	$ 264,166,000